1. Chapter 37
Placenta Previa, Placenta Accreta,
Abruptio Placentae, and Vasa Previa
Andrew D. Hull, MD, and Robert Resnik, MD
Bleeding in the later stages of pregnancy has been described as “third- the cervix dilates or the lower uterine segment effaces. The term “low-
trimester bleeding” or “antepartum hemorrhage.” Late pregnancy lying placenta” should be reserved for cases in which the placenta is
bleeding is a significant cause of maternal and fetal morbidity, fetal seen on transabdominal ultrasound to extend into the lower uterine
mortality, and preterm delivery. Traditional accounts of such bleeding segment but its precise limits have not been defined, and for cases
have addressed placenta previa, abruptio placentae, and vasa previa, identified before the third trimester. TVUS allows location of the pla-
although in fact the only thing that these clinical problems have in centa in relation to the internal cervical os with great precision. When
common is that they all are concerned, to a greater or lesser extent, such studies are performed, the placenta may be classified as a complete
with hemorrhage. The etiology, management, and complications of previa if it completely covers the internal os. The term marginal pla-
each are quite distinct. In the past, uncertainty in precise diagnosis of centa previa should be used if the placental edge lies 2.5 cm or closer
the cause of late pregnancy bleeding has led to these conditions being to the internal os. It has been shown that when the placenta is more
considered together, but the universal availability of ultrasound tech- than 2 to 3 cm from the cervix, there is no increased risk of bleeding.2
nology has eliminated much of the diagnostic dilemma. A definitive diagnosis of placenta previa should be avoided in asymp-
Bleeding during the second half of pregnancy complicates about tomatic patients before the third trimester, because many cases of pla-
6% of all pregnancies. Placenta previa is ultimately documented in 7% centa previa identified early in pregnancy will resolve as pregnancy
of cases, and evidence of significant placental abruption is found in advances.
13%. In the remaining 80% of cases, the bleeding can be ascribed to Placenta previa affects about 1 (0.5%) of every 200 pregnancies at
either early labor or local lesions of the lower genital tract or no source term.3 There is some evidence that the incidence of placenta previa is
can be identified.1 Faced with a woman with late pregnancy bleeding, increasing.4 This increase may be related to the increasing rate of
the clinician must rapidly reach a firm diagnosis and management plan cesarean section observed in all developed countries. A single prior
to ensure the optimum outcome for mother and baby. Ultrasonogra- cesarean section or a prior pregnancy complicated by placenta previa
phy, electronic fetal monitoring, and, frequently, evaluation of the increases the incidence of placenta previa in a subsequent pregnancy
function of the maternal coagulation system make up the foundation to as high as 5%,1,5,6 rising even further with a history of more prior
on which both diagnosis and management are developed. Clinical cesarean deliveries.7 Advanced maternal age increases the incidence of
assessment must occur simultaneously with imaging and fetal placenta previa to 2% after 35 years of age and 5% after age 40.7 Mul-
assessment. tiparity, prior suction curettage, and smoking are all associated with
In asymptomatic patients who are without antenatal bleeding but higher risks of placenta previa.8-10 The relative risks for these associated
have been identified by prenatal ultrasound as having risk factors (pla- factors are summarized in Table 37-1.
centa previa, placenta accreta, or vasa previa), timing of delivery is the
most important clinical decision that has to be made.
Pathogenesis
The underlying cause of placenta previa is unknown. There is a clear
association between placental implantation in the lower uterine
Placenta Previa segment and prior endometrial damage and uterine scarring from
curettage, surgical insult, prior placenta previa, and multiple prior
Definition and Epidemiology pregnancies.
Advances in the precision of sonographic diagnosis, particularly trans- At least 90% of placentas identified as being “low lying” in early
vaginal ultrasound (TVUS) technology, as well as an increased under- pregnancy will ultimately resolve by the third trimester.11 The term
standing of the changing relationship between the placenta and the “placental migration” is widely used to describe this phenomenon. The
internal cervical os as pregnancy advances, have rendered traditional placenta clearly does not move; rather, it is likely that the placenta
definitions and classifications of placenta previa obsolete. Placenta grows toward the better blood supply at the fundus, a process known
previa exists when the placenta covers the cervix either completely or as trophotropism, leaving the distal portions of the placenta, closer to
partially or extends close enough to the cervix to cause bleeding when the relatively poor blood supply of the lower segment, to regress and

2. 726 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa
atrophy. As the uterus grows and expands to accommodate the devel- onset of labor. Some women experience pain secondary to uterine
oping fetus, there is differential growth of the lower segment, and this contractions. Bleeding may be provoked by labor, examination, or
may further increase the distance between the lower edge of the pla- intercourse but it usually has no identifiable precipitating cause. The
centa and the cervix. patient is more likely to have a fetus with an abnormal lie, inasmuch
Bleeding from placenta previa may occur before labor as a result of as the placenta previa may prevent the fetus from establishing normal
development of the lower uterine segment and effacement of the cervix polarity. All women presenting with painless vaginal bleeding after 20
with advancing gestation. Prelabor uterine contractions may also weeks’ gestation should be assumed to have a placenta previa until
produce bleeding, as may intercourse or injudicious vaginal examina- proven otherwise. Transabdominal ultrasound should be quickly uti-
tion. Once labor begins, significant bleeding will occur as the cervix lized to screen for placenta previa. Unless the placenta is clearly fundal
dilates and the placenta is forced to separate from the underlying and the lower segment is clear, TVUS should then be performed.
decidua. Transabdominal ultrasound has been shown to be inferior to TVUS
for definitive placental localization.12,13 Concerns regarding the poten-
tial for TVUS to provoke bleeding are unfounded, and several studies
Diagnosis have confirmed the safety of a careful TVUS approach (Fig. 37-1).7,13,14
The classic history for placenta previa is that of painless third-trimester The placement of the transvaginal probe should be observed continu-
bleeding. Several small “herald bleeds” may occur in advance of major ously on the ultrasound monitor during insertion, to avoid placing the
hemorrhage, but in up to 10% of cases there is no bleeding until the probe into a potentially dilated cervix. If a transvaginal probe is
unavailable, translabial imaging using a regular abdominal probe can
produce excellent results, with better visualization of the relationship
TABLE 37-1 RISK FACTORS AND RELATIVE between the cervix and placenta than is obtained from transabdominal
scanning.15 A digital or speculum examination to inspect the cervix for
RISKS OF PLACENTA PREVIA
local causes of bleeding should not be performed until placenta previa
Risk Factor Increased Risk Reference has been excluded by ultrasonography.
In the unusual setting of significant late pregnancy bleeding where
Previous placenta 83× Monica and Lilja, 1995 ultrasound is not available and the diagnosis is not clear, there is still
previa
a place for the “double-setup” examination. The patient is taken to the
Previous cesarean 1.5-153× Herschkowitz et al, 1995;
section Hemminki and
operating room, where preparations are made for a cesarean delivery.
Merilainen, 1996 A vaginal examination is then performed, beginning in the vaginal
Previous suction 1.33× Taylor et al, 1994 fornices and avoiding placing the fingers directly in the cervix. If a
curettage for placenta previa is detected, cesarean section is then performed. If no
abortion placenta previa is found, a search for other causes of third-trimester
Age > 35 yr 4.73× Iyasu et al, 1993 bleeding ensues.
Age > 40 yr 93× Ananth et al, 1996
Multiparity 1.1-1.73× Williams and Mittendorf, Implications of Early Pregnancy Diagnosis
1993 The routine use of ultrasonography in the first and second trimesters
Nonwhite race (all) 0.33× Iyasu et al, 1993
of pregnancy has led to the frequent observation of a low-lying pla-
Asian race 1.93× Iyasu et al, 1993
Cigarette smoking 1.4-33× Handler et al, 1994; centa or a previa.
Ananth, Savits, and Transabdominal ultrasound tends to over-diagnose low-lying pla-
Luther, 1996; Chelmow centa, especially when the bladder is empty.16 Even with TVUS, the
et al, 1996 findings may not correlate with the placental position at term. Several
reports confirm that up to 10 times as many women are found to have
B
A
FIGURE 37-1 Ultrasound study performed at 18 weeks’ gestation for fetal anatomy survey. A, Transabdominal ultrasound shows an
apparent “low-lying placenta.” B, Transvaginal ultrasound shows that the placenta completely covers the cervix.

3. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 727
a placenta previa in the second or first trimester than at delivery.14,17-21 the less likely it is that a significant prolongation of pregnancy will be
The earlier in pregnancy a diagnosis of placenta previa is made, the gained.26 Betamethasone to enhance fetal lung maturation should be
less likely it is that the finding will persist at delivery.22 The likelihood administered to patients who are at less than 34 weeks’ gestation if
of persistence to term of a placenta previa found in the second trimes- expectant management is planned.
ter is also related to the degree to which the placenta overlies the There is controversy regarding the role of tocolytics in the setting
cervix14,17-21 and the thickness of the placental edge.23 It is recom- of hemorrhage from placenta previa. Both β-mimetics and magnesium
mended that a follow-up ultrasound study be performed between 28 sulfate27-29 have been used in this setting and appear to be associated
and 32 weeks’ gestation to further evaluate the placental position. If with significant prolongation of pregnancy without adverse effects.
there appears to be a significant change in the position of the placental After the initial presentation with bleeding, patients should remain
edge over time, a final study should be done at 36 weeks, before making in hospital until they are free of bleeding for at least 48 hours. Some
a final decision as to the appropriate route of delivery. may then be considered for home management. Several studies have
addressed the issue of safety of outpatient management in a controlled
setting at home.25,30-33 With the exception of one report of an increase
Management in perinatal mortality and morbidity and earlier gestational age,34 it
Any woman with vaginal bleeding after 20 weeks’ gestation should be appears to be a safe approach. Patients selected for home management
assessed on a labor and delivery unit. The primary focus should be on should be asymptomatic with regard to bleeding and abdominal pain,
hemodynamic assessment of the mother and assessment of fetal well- be able to remain at home with limited activity, and have adequate
being. Vital signs are obtained, and electronic fetal monitoring initi- support as well as adequate access to transport to a nearby hospital if
ated. One or two large-gauge intravenous lines should be placed, and bleeding recurs. A second significant bleeding episode usually results
maternal blood should be sent for determination of the hematocrit in readmission until delivery.
and type and screen. For substantial bleeding episodes, 2 to 4 units of Several strategies have been proposed to reduce the risk of hemor-
blood should be cross-matched. Obstetric units might consider the use rhage in women with a known placenta previa. Bed rest, reduced
of an “Obstetric Hemorrhage Protocol” to facilitate access to the activity, and avoidance of intercourse are commonly mandated and
resources of the hospital blood bank for this and any other obstetric seem logical, although there is no conclusive evidence to support these
hemorrhage (Table 37-2). Rh immune globulin is administered, when measures. Cervical cerclage was evaluated in two small prospective
appropriate, to Rh-negative, nonimmunized women. studies35,36 without clear benefit and is not recommended.
Once the patient is stabilized and fetal condition has been assessed, All women whose placenta lies within 2 cm of the cervix, as docu-
the definitive cause of the bleeding can be addressed. If the diagnosis mented by a late third-trimester TVUS scan, should be delivered by
is clearly placenta previa and the patient is at or beyond 36 weeks’ cesarean section.37-39 An asymptomatic woman whose placenta lies
gestation, delivery is appropriate. If bleeding is excessive or continues, more than 2 cm from the cervical os can be allowed to labor safely.16
or if there are concerns about the condition of the fetus, the patient It should be noted that the presence of a low-lying placenta, even if it
should be delivered regardless of gestational age. In all other cases, does not cause intrapartum bleeding, increases the risk of postpartum
management may be conservative and has been shown to be safe,24,25 hemorrhage because of lower uterine segment atony.
with prolongation of pregnancy by an average of 4 weeks after the Cesarean section for placenta previa should be performed by the
initial bleeding episode. The closer it is to a gestational age of 36 weeks, most experienced team available because of the substantial risk of
intraoperative hemorrhage.40 In most instances, a lower uterine
segment incision is appropriate. If the placenta is anterior, it is neces-
sary to clamp the umbilical cord immediately to prevent excessive
TABLE 37-2 OBSTETRIC HEMORRHAGE blood loss caused by disruption of the placenta during entry. A vertical
PROTOCOL incision is also reasonable in such cases and may be preferable if the
Blood is immediately drawn and set up for
fetus is premature or if a transverse lie exists.41 Postpartum hemor-
Type and cross-matching rhage may occur from the placental implantation site secondary to
Hematocrit atony and may require the use of additional pharmacologic agents to
Coagulation studies (PT/PTT/fibrinogen) control blood loss, such as methylergonovine maleate (Methergine),
Wall clot (blood is drawn into a plain tube and set aside—should 15-methyl prostaglandin F2α (Hemabate), and high-dose oxytocin,
clot within 6 min) used either singly or in combination. The B-Lynch suture42 or local
An ABG determination may be requested to assess acute blood suturing of the placental bed may be needed to control bleeding. In
loss (typically, every increment in base deficit of −1 to −2 rare cases of refractory hemorrhage, hysterectomy may be required.
requires 1 unit of PRBCs to correct it) Among women known to have a placenta previa who do not require
Four units of type-specific or O-negative blood are made
very early delivery, elective delivery should be performed before sig-
immediately available.
The laboratory immediately starts to cross-match 4 units of blood
nificant bleeding has occurred. It is reasonable to plan on delivery at
and stays 4 units ahead of blood use. or just after 36 weeks’ gestation, because there is little fetal advantage
Two units of FFP are thawed and made available. after that time, when weighed against the risk of a sudden and possibly
One 10-pack of platelets is made available. excessive bleeding episode. The alternative is to perform an amniocen-
The blood bank is alerted to provide further units of blood, FFP, tesis to confirm lung maturity before delivery, but the risk of hemor-
and platelets as needed. rhage with delayed delivery usually outweighs the risk of fetal lung
Further samples for ABGs and other laboratory studies are drawn immaturity at that gestational age.
as required. The selection of anesthesia to be used for cesarean section in cases
ABGs, arterial blood gases; FFP, fresh-frozen plasma; PRBCs, packed
of placenta previa should be decided by the obstetrician and anesthe-
red blood cells; PT, prothrombin time; PTT, partial thromboplastin siologist involved with the delivery, in concert with the patient. In the
time. United Kingdom, regional anesthesia was preferred by most obstetric

4. 728 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa
anesthesiologists in a survey43 and was used in 60% of cases in a ret- with placenta previa and prior uterine surgery.46,47 In patients with
rospective series.44 Regional anesthesia is associated with lower opera- placenta previa, the risk of accreta is 10% to 25% with one prior cesar-
tive blood loss and less need for transfusion than general anesthesia,3,45 ean section and exceeds 50% with two or more prior cesareans.48-50
probably because many inhaled anesthetics cause uterine relaxation. Prevalence appears to be similar in women with these risk factors
undergoing second-trimester pregnancy termination.51
The diagnosis of placental invasion of the myometrium usually can
Complications be made by ultrasound,52-54 with a reported sensitivity and specificity
for the diagnosis of approximately 0.8 and 0.95, respectively.53,55,56 Mag-
Placenta Accreta netic resonance imaging (MRI) has also been used to confirm the
One of the most serious complications of placenta previa is the devel- diagnosis or better delineate the presence or extent of accreta.53 MRI
opment of placenta accreta. This condition involves trophoblastic inva- is also useful in the presence of a posterior placenta and in the assess-
sion beyond the normal boundary established by the Nitabuch fibrinoid ment of deep myometrial, parametrial, and bladder involvement.55,57
layer. If invasion extends into the myometrium, the term placenta The ultrasound appearance of a normal placental attachment site
increta is used; placental invasion beyond the uterine serosa (at times is shown in Figure 37-3A. Normal attachment is characterized by a
involving the bladder or other pelvic organs and vessels) is termed homogeneous appearance of the placenta and a hypoechoic boundary
placenta percreta. Histologic examples of normal placental implanta-
tion and placenta accreta are shown in Figure 37-2.
Placenta accreta is associated positively with advanced maternal
age, smoking, and parity, but the strongest recognized association is
B
P
C
P
A
B
P
C
B
FIGURE 37-3 Ultrasound appearance of a normal placental
attachment site and placenta accreta. A, Normal placental
attachment in an anterior placenta previa. A hypoechoic area
FIGURE 37-2 Histologic appearance of normal placental separates the bladder wall and the placental tissue, representing
implantation and placenta accreta. A, Histologic section of a myometrium and myometrial vasculature. B, Characteristic ultrasound
normal placental attachment site. Trophoblastic tissue with anchoring appearance of placenta accreta. Note the lack of a hypoechoic area,
villi encroach but do not go through the Nitabuch membrane. as well as obliteration of the well-delineated bladder wall. In addition,
B, Representative histologic section of placenta accreta, there are intraplacental sonolucent spaces (arrows) adjacent to the
demonstrating invasion of trophoblasts into the myometrial tissue. involved uterine wall. B, bladder; C, cervix; P, placenta.

5. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 729
between the placenta and the bladder that represents the myometrium
and the normal retroplacental myometrial vasculature. The bladder
wall is intact throughout. In contrast, placental accreta is associated Vasa Previa
with loss of the normal hypoechoic boundary, and there are usually
intraplacental sonolucent spaces adjacent to the involved uterine wall Definition and Epidemiology
(see Fig. 37-3B). Color-flow and power Doppler sonography have also Vasa previa is a rare but potentially catastrophic complication in which
been reported to facilitate the diagnosis.58-60 Chou and associates60 fetal vessels run through the fetal membranes and are at risk of rupture
evaluated 80 women with placenta previa to determine the accuracy with consequent fetal exsanguination. It is estimated that vasa previa
of color-flow Doppler ultrasonography in distinguishing between affects between 1 in 1275 and 1 in 8333 pregnancies.73,74
uncomplicated placenta previa and placenta accreta. Using their crite-
ria, the antepartum diagnosis of accreta was made in 16 of the 80
women studied and was confirmed histopathologically in 14. The sen- Pathogenesis
sitivity and specificity for diagnosis were 0.82 and 0.97, respectively. Vasa previa may occur because the insertion of the umbilical cord into
Although it is clear that larger numbers of patients must be studied by the placenta is velamentous, with the umbilical vessels coursing through
these various modalities to more accurately determine the sensitivities the fetal membranes before inserting into the placental disk and the
and specificities of diagnosis, various types of ultrasonography and unsupported vessels then overlying the cervix. It may also result from
MRI appear to hold promise in making or excluding the diagnosis in the presence of a bilobed or succenturiate placenta with the vessels
most cases. connecting the placenta similarly overlying the cervix.16 If the condi-
It is important, if at all possible, to make the diagnosis before deliv- tion goes unrecognized, it is associated with a fetal mortality rate of
ery, because intraoperative hemorrhage can be massive, and placenta almost 60%. In addition to a succenturiate placenta75 and velamentous
accreta has been reported to be the most common indication for emer- insertion, other risk factors include a low-lying placenta observed in
gency peripartum hysterectomy.61,62 In an effort to diminish blood loss, the second trimester,76 multiple gestation, and in vitro fertilization.77
it is recommended that delivery be accomplished through a fundal
incision followed by clamping of the cord. The placenta is allowed to
remain in situ while the surgeons proceed to a total abdominal hyster- Diagnosis
ectomy. This may require very complex surgical technique and plan- The key to reducing fetal loss from vasa previa is prenatal diagnosis.16
ning, and a pelvic surgeon capable of wide resection of the lower Many cases of vasa previa are identified only at the time of vessel
uterine segment and parametrial areas should be available, as well as rupture in labor. Vaginal bleeding is followed by fetal distress and death
ample transfusion capability. if emergent delivery cannot be effected in time. Because the entire fetal
Although published reports are not extensive, it has been suggested cardiac output passes through the cord, it can take less than 10 minutes
that balloon occlusion of the aorta or internal iliac vessels may help to for total exsanguination to occur. Electronic fetal monitoring may
prevent excessive blood loss during resection of the lower uterine show an initial tachycardia, rapidly followed by decelerations, brady-
segment. This involves preoperative placement of balloon-tipped cath- cardia, and a preterminal sinusoidal rhythm.78 If a cesarean delivery
eters retrograde through the femoral arteries immediately before can be accomplished immediately and with sufficient rapidity, good
surgery. The catheters are guided under fluoroscopic direction into the newborn outcome can be obtained by aggressive postnatal transfusion
internal iliac arteries and inflated during the dissection. However, the therapy.79
value and safety of this approach have been challenged with the recent It has been suggested that the blood from the vagina may be tested
reports of no proven benefit and embolic complications.63,64 Neverthe- to confirm its fetal origin, using the Apt or Kleihauer-Betke tests or
less, the placement of catheters does provide the opportunity to manage electrophoresis.80 In actual practice, such tests are either unavailable or
potential postoperative bleeding with angiographic embolization cannot be done quickly enough to be of any value. Occasionally, fetal
rather than reexploration. vessels have reportedly been felt through the membranes during
Conservative management may be an option if there is suspicion vaginal examination or visualized on amnioscopy; such observations
of a small focal accreta, if there is a fundal location after a myomectomy are really only of historical interest in modern practice.
or classic cesarean section, or with a posteriorly implanted placenta. A It is now well established that vasa previa may be diagnosed prena-
few reports have suggested leaving the uterus and placenta in situ and tally using ultrasound.81,82 Routine obstetric ultrasound should include
using methotrexate postoperatively.65-67 However, the numbers of an assessment of the placental site and number of placental lobes and
reported cases are very few, and hemorrhagic and infectious complica- an evaluation of the placental cord insertion site. In all cases in which
tions have usually resulted. Consequently, a definitive surgical approach a multilobed or succenturiate placenta or a low-lying placenta or vela-
to this serious obstetric complication is strongly recommended. mentous cord insertion is identified using transabdominal ultrasound,
a detailed examination of the lower uterine segment and cervix should
Neonatal Complications be performed using TVUS. Gray-scale ultrasound can identify placen-
It has been suggested that repetitive bleeding from placenta previa is tal cord insertion in most cases, but color or power Doppler makes the
associated with fetal growth impairment,68 although this has been dis- process easier and should be used (Figs. 37-4 and 37-5).81-84 There have
puted.26 Pregnancies complicated by placenta previa have also been been several studies evaluating this approach for prenatal detection of
reported to be associated with higher rates of fetal anomalies,69 neuro- vasa previa,81,82,84,85 all of which showed high specificity and sensitivity
developmental delay,70 and sudden infant death syndrome (SIDS).71 of detection with little impact on the length of scan time. More impor-
The reasons for these findings are unknown. tantly, in cases in which vasa previa was detected prenatally, there were
As might be expected, placenta previa and previa accreta are a cause no fetal deaths from the condition. A recent retrospective, multicenter
of preterm birth due to the need for iatrogenic preterm delivery. study showed newborn survival rates of 97% in prenatally detected
Accreta has also been reported to have a negative influence on fetal cases of vasa previa and a fetal loss rate of 56% in cases not identified
growth (odds ratio, 5.05 compared with controls).72 before the commencement of labor.86 Newer imaging modalities such

6. 730 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa
B
A
D
C
FIGURE 37-4 Vasa previa identified at 18 weeks’ gestation on routine ultrasound studies. A, Transabdominal power Doppler identifies
the umbilical cord possibly overlying the cervix. B, A fetal arterial waveform using power and pulse-wave Doppler. C, Vasa previa in gray
scale on transvaginal ultrasound. D, Confirmation of the vasa previa using color Doppler transvaginally.
because the vessels may be compressed by the presenting part and thus
difficult to visualize.
Management
There is no uniformity of opinion as to the optimal management
strategy for pregnancies with a prenatally diagnosed vasa previa, par-
ticularly in regard to the timing of elective delivery. It has been sug-
gested that patients be hospitalized at 30 to 32 weeks and delivered at
35 to 36 weeks’ gestation without confirmation of lung maturity by
amniocentesis.16 This approach is based on the 10% risk of membrane
rupture before labor and the high associated fetal mortality rate. Our
approach has been to assess cervical length weekly from at least 30
weeks using TVUS. If the cervix is 2.5 cm in length or greater, out-of-
FIGURE 37-5 Vasa previa. The placenta from the case identified in
hospital management continues. The patient is administered beta-
Figure 37-4. The patient was delivered by elective cesarean section at methasone just before 34 weeks’ gestation and is delivered by cesarean
34 weeks. Arrow shows velamentous cord insertion. section between 34 and 35 weeks, without additional testing for fetal
lung maturity.
as MRI87 and three-dimensional ultrasound have been described in the
evaluation of vasa previa.87-89 MRI is of little practical use in routine Prevention of Adverse Outcomes
cases. Although transvaginal three-dimensional power Doppler pro- A significant reduction in fetal mortality should be possible with a
vides an excellent means of visualizing the entire lower uterine segment diligent search as previously described. Public and professional aware-
for the evaluation of vasa previa, similar information may be obtained ness has been heightened by such organizations as The International
by careful use of a two-dimensional vaginal probe. Such imaging com- Vasa Previa Foundation (http://www.vasaprevia.com [accessed Febru-
bined with maternal positional change, the use of the Trendelenburg ary 8, 2008]). However, a high index of suspicion by the attending
position, and gentle manual elevation of the fetal presenting part aid physician and a meticulous approach to diagnosis provide the best
in visualizing the fetal vessels. The latter technique is particularly useful opportunity for a favorable outcome.

7. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 731
of fetal nutrition. The process may be self-limited or ongoing with
further dissection of the decidua. Dissection can lead to external bleed-
Abruptio Placentae ing if it reaches the placental edge and tracks down between the fetal
membranes; circumferential dissection leading to near-total separation
Definition and Epidemiology of the placenta can occur, particularly with concealed abruption. The
Abruptio placentae is the premature separation of a normally sited underlying event in many cases of abruption is thought to be vaso-
placenta before birth, after 20 weeks’ gestation. It is a particularly haz- spasm of abnormal maternal arterioles. Some cases may result from
ardous condition associated with significant maternal and fetal mor- venous hemorrhage into areas of the decidua that have become necrotic
bidity and mortality. About 1% of all pregnancies are complicated by secondary to thrombosis. Long-standing predisposition to abruption
clinically recognized abruption.90-93 The degree of abruption ranges may be inferred from the finding that women destined to suffer abrup-
across a broad clinical spectrum, from minor degrees of placental tion have low levels of pregnancy-associated plasma protein A (PAPP-
separation, with little effect on maternal or fetal outcome, to major A).105 Evidence of preexisting placental pathology in women with
abruption associated with fetal death and maternal morbidity. Abrup- abruption includes poor trophoblastic invasion,106 inadequate remod-
tion sufficient to cause fetal death occurs in about 1 of every 420 eling of the uterine circulation as reflected by abnormal uterine artery
deliveries.94 If placentas are routinely examined after delivery, evidence Doppler flow,107 and the well-established associations among pre-
of abruption may be found in almost 4% of cases, most of which were eclampsia, IUGR, and abruption—all of which may be regarded as
unrecognized and of no apparent clinical consequence. There has been primary placental disorders. Abruption may also occur secondary to
an increase of almost 25% in the rate of clinically detected abruption acute shearing forces affecting the placenta-decidua interface, such as
in the United States in recent decades, with a disproportionate increase those that occur with trauma—particularly rapid deceleration injuries
seen among African-American women.95 (motor vehicle accidents) and the sudden decompression of an over-
The incidence of abruption peaks between 24 and 26 weeks’ gesta- distended uterus that occurs with membrane rupture in polyhydram-
tion.96 Approximately 10% of all preterm births occur because of nios or delivery of a multiple gestation.
abruption,90 and the infant outcomes are associated with increased As the abruption process continues, loss of placental function
rates of perinatal asphyxia,97 intraventricular hemorrhage, periven- results in fetal hypoxia and may end in fetal death. The acute hemor-
tricular leukomalacia,98 and cerebral palsy99 when compared with ges- rhage activates the coagulation cascade, and, with ongoing bleeding,
tational age–matched controls. Perinatal mortality in pregnancies disseminated intravascular coagulation (DIC) may result. Continued
complicated by abruption may be declining overall,93,100 but the rate bleeding, with maternal hypovolemia and poor tissue perfusion, aggra-
continues to be higher than in gestational age–matched controls vates the DIC and results in a downward spiral into hemorrhagic
without abruption.96 Placental separation is strongly associated with shock. Bleeding into the myometrial tissue can lead to a Couvelaire
preterm premature rupture of the membranes (pPROM), in both a uterus which becomes atonic and increases the risk of uterine hemor-
causal and a consequential manner.101 Most pregnancies complicated rhage after delivery.
by abruption result in the delivery of an infant weighing less than the
10th percentile for gestational age,102-104 suggesting a common pathway Risk Factors and Associations for Abruption
linking abruption to placental dysfunction and intrauterine growth The most important risk factor for abruption is a history of abruption
retardation (IUGR). in a prior pregnancy.108 One meta-analysis showed an increase of up
to 20-fold in the risk of abruption if a prior pregnancy had been simi-
larly affected.109 With two prior pregnancies complicated by abruption,
Pathogenesis the risk of recurrence is 25%.94
Abruption results from bleeding between the decidua and placenta Maternal hypertension is also a significant risk factor for abruption.
(Fig. 37-6). The hemorrhage dissects the decidua apart, with loss of Chronic hypertension is associated with a fivefold increase in risk,
the corresponding placental area for gaseous exchange and provision which rises to eightfold with superimposed preeclampsia.96 Preeclamp-
sia alone is also strongly linked to abruption and to the severity of
abruption.110 It seems plausible that preeclampsia and abruption share
many underlying pathologic mechanisms.
Perhaps the most readily preventable risk factor for abruption is
cigarette smoking. Cigarette smokers are up to 2.5 times more likely to
have an abruption than nonsmokers,111-115 and they have twice the
perinatal mortality of nonsmokers.116 There is a dose-response rela-
tionship between the number of cigarettes smoked and the risk of
abruption.115,117,118 Even women who stop smoking before pregnancy
are at increased risk. Substance abuse is closely linked to abruption—
any agent that causes vasospasm or transient severe hypertension may
be causative.119,120 In the United States, as many as 10% of pregnant
women who are cocaine and crack cocaine users will experience pla-
cental abruption.121 Multiparity is also positively correlated with a
small increase in the risk of abruption.1,102 The apparent association
between maternal age and abruption is not significant when parity is
taken into account.
There has been substantial recent interest in the possible association
FIGURE 37-6 Abruptio placentae. A large retroplacental abruption between thrombophilic disorders and abruption. Some retrospective
at 30 weeks’ gestation is shown. studies of abruption have found increased rates of thrombophilia.122,123

8. 732 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa
However, both retrospective124 and prospective case-control studies125 hemorrhage. Fetal compromise is a common finding, and if more than
of women with the factor V Leiden mutation showed no increase in 50% of the placenta is involved, fetal death is likely. Massive concealed
abruption risk. It is established that hyperhomocysteinemia is associ- abruption often manifests with severe pain, a hard uterus, and a dead
ated with abruption,123 although, in the absence of hyperhomocystein- fetus; such a picture may occur in association with severe preeclampsia
emia, the specific MTHFR gene mutations themselves do not appear or the recent use of a vasoactive drug such as cocaine.139 If abruption
to be associated with an increased risk.126,127 occurs in a posteriorly located placenta, severe back pain may be the
Motor vehicle accidents are the most common traumatic event only symptom; it may be worsened by abdominal palpation that pushes
associated with abruption, and clinical evidence of abruption may not the fetus against the placenta. Abruption may precipitate preterm
be apparent for 24 hours or longer after the trauma. Women with labor, and it should always be considered in the differential diagnosis
vaginal bleeding or contractions after a motor vehicle accident should for a patient in apparent idiopathic preterm labor.
be observed for at least 24 hours; those who are asymptomatic can Although ultrasonography is an integral part of the diagnostic
safely be discharged after 6 hours of monitoring.128,129 approach to late pregnancy bleeding, its utility is primarily for the
Membrane rupture may precede or follow chronic retroplacental exclusion of placenta previa as the cause of hemorrhage. At least 50%
bleeding or an acute abruption, and women with ruptured membranes of abruptions produce no findings on ultrasound.140-142 What is visual-
should be monitored carefully for this possibility.109,130 Those with ized by ultrasound depends on the site, scale, and timing of bleeding.
early pregnancy bleeding who have a subchorionic hematoma visible In early acute abruptions, blood and clot retained within the uterus
on ultrasound are also at increased risk for both pPROM and appear as hyperechoic or isoechoic collections relative to placental
abruption.131 echogenicity.143 In cases that remain undelivered, the hematomas
Screening tests performed for other indications may identify groups resolve over several weeks, becoming hypoechoic and then sonolucent,
of women who are at increased risk for abruption but may have no usually by 2 weeks after the event.143 Intrauterine clot may “jiggle”
other high-risk factors. These tests include maternal first- and second- when bounced by the transducer—the “jello” sign. An acute abruption
trimester serum screening for aneuploidy. Women with PAPP-A levels with obvious vaginal bleeding, in which little or no blood is retained
below the 5th percentile at the time of first-trimester serum screening within the uterus, may have no specific sonographic findings. There-
for trisomy 21 have an increased risk of abruption,105 but low levels of fore, the absence of ultrasound findings never excludes an abruption.
human chorionic gonadotropin (hCG) in the first trimester are not Cardiotocography is an integral part of the evaluation for late preg-
similarly associated.132 In one study of routine uterine artery Doppler nancy bleeding. Abruption is commonly accompanied by uterine con-
velocimetry performed at 11 to 14 weeks’ gestation as a screen for tractions that may not be appreciated clinically, particularly after
IUGR and preeclampsia, a pulsatility index higher than the 95th per- trauma.144 Fetal heart rate tracings may exhibit a variety of abnormal
centile or a PAPP-A value lower than the 10th percentile predicted 43% patterns, including variable and late decelerations, poor variability,
of pregnancies with a subsequent abruption.133 In an earlier study of prolonged bradycardia, or a sinusoidal pattern; these are not specific
uterine artery Doppler ultrasound, persistent notching of the wave- to abruption and reflect underlying evolving fetal asphyxia.
form after 24 weeks was associated with increased risk for abruption Kleihauer-Betke testing is of no diagnostic value in abruption: It
as well as IUGR and preeclampsia.107 may be negative with proven abruption128,129,145 or positive when no
Women with otherwise unexplained elevated serum levels (>2 mul- abruption has occurred. Its only value in this setting is to guide Rh
tiples of the median [MOM]) of α-fetoprotein (AFP) on second-tri- immune globulin dosing in Rh-negative women who are thought to
mester serum screening for trisomy 21 have long been thought to be have sustained an abruption.
at increased risk for a wide range of adverse pregnancy outcomes, Chronic abruption-oligohydramnios sequence (CAOS) is a term
including abruption.134,135 However, a recent case-matched, prospective that was coined to describe women who present with bleeding attrib-
study found elevated AFP levels to be associated with an increased risk uted to abruption and go on to develop oligohydramnios without
of abruption but no increase in the frequency of IUGR, preterm deliv- evidence of ruptured membranes.146 Twenty-four patients were
ery, low birth weight, or fetal death.136 A recent attempt to establish a described, all of whom delivered preterm (average gestational age, 28
critical cutoff value for elevated AFP and increased risk of abruption weeks). For the most part, the earlier the onset of bleeding, the earlier
stressed the low specificity and high false-positive rate of the test.136 the delivery. More than half of the women went on to develop pPROM
Elevated hCG values at the time of second-trimester serum screen- before delivery and after the development of oligohydramnios.
ing have similarly been associated with adverse pregnancy outcome,
including abruption.137 Previously, a value greater than 2.0 MOM was
thought to be significant in this context, but one recent case-controlled Management
study showed that the threshold should be set at 3.0 MOM. Even at The key to optimizing maternal and fetal outcome in abruptio placen-
that level, a positive test had poor predictive value and was not associ- tae is the individualization of care. Precise management depends on
ated with increased risk of abruption.138 Abnormal inhibin values do the extent of maternal and/or fetal compromise and the gestational
not appear to be predictive of abruption. age. Decision making should be rapid but methodical; delay in diag-
nosis and inappropriate triage leads to significantly increased perinatal
mortality.147 Twenty percent of all fetal deaths from abruption occur
Diagnosis after presentation to the hospital, and 30% of those deaths occur
The diagnosis of placental abruption is made based on clinical find- within 2 hours after admission.
ings. The classic presentation is that of vaginal bleeding, usually Initial assessment should focus on maternal hemodynamic status
accompanied by abdominal (uterine) pain. Examination often reveals (remembering that blood pressure may be elevated in the setting of
uterine tenderness, and contractions may be present. About 10% of preeclampsia) and fetal well-being. Maternal vital signs should be mea-
abruptions are concealed, with no vaginal bleeding. If bleeding is sured frequently, because they may change suddenly as the abruption
present, the amount is often a poor guide to the degree of separation, evolves. Electronic fetal monitoring should begin immediately and be
because there is usually a mixed picture of apparent and concealed continuous throughout further assessment and management. A large-

9. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 733
gauge intravenous line should be placed (two lines if the patient is If the patient has sustained a mild separation at a premature gesta-
hemodynamically unstable). Initial laboratory studies should include tional age and is asymptomatic without evidence of bleeding, discharge
a baseline complete blood count and platelet count, type and screen home may be considered as an alternative to prolonged hospitalization.
and cross-match where appropriate, blood urea nitrogen and electro- Either way, a clear management plan for delivery should be developed
lytes, coagulation studies, and a wall clot. These studies serve as useful based on subsequent events or the reaching of an arbitrary gestational
baseline references. An indwelling bladder catheter should be placed to age (usually 37 weeks). The evaluation of patients undergoing expect-
allow urinary output to be closely monitored. In unstable or critically ant management should include regular assessment of fetal growth
ill patients, management may be aided by placement of a central and tests of well-being, because these fetuses are at increased risk for
venous pressure line (preferably with a Cordis introducer) or an arte- IUGR.
rial line. The involvement of the obstetric anesthetic team should be If an abruption occurs after 34 weeks’ gestation and maternal
sought early. and fetal condition permit, vaginal delivery is preferred. Amniotomy
After these steps are taken, attention should be directed at excluding should be performed, and, if needed, an oxytocin infusion should
a placenta previa (by ultrasound examination) and deciding on the be started. Labor usually progresses rapidly, even without augmenta-
timing and route of delivery. Maternal or fetal compromise mandates tion. However, if progress is slow or maternal or fetal status deterio-
immediate delivery, usually by cesarean section unless the patient is in rates, cesarean section should be performed. If abruption has resulted
an advanced stage of labor. If the event occurs after 34 weeks’ gestation, in fetal death, vaginal delivery is preferred unless there are other
delivery should not be delayed, because the risks of conservative man- obstetric contraindications or the mother is hemodynamically
agement outweigh any considerations of prematurity of the fetus. unstable.
Between 20 and 34 weeks, if mother and fetus are stable, an attempt at Coagulopathy develops in about 10% of abruptions. It usually is
conservative management may be considered.148,149 Betamethasone related to the severity of the event and is particularly likely to occur if
should be administered to enhance fetal lung maturity in all such cases. there is fetal demise or massive hemorrhage. An aggressive approach
The patient should be monitored closely, because she continues to be should be used to maintain maternal blood volume and oxygen-
at risk of an evolving process. The use of tocolytics is controversial; in carrying capacity, including the use of component therapy (fresh-
most cases, they should not be used. Although studies addressing this frozen plasma and platelets). The coagulation tests most frequently
issue have found no increase in adverse fetal or maternal events,27,141,148-150 used, and the component replacement therapy for women with DIC,
no prospective trial has been performed. are summarized in Tables 37-3 and 37-4.
TABLE 37-3 COAGULATION TESTS USED IN THE DIAGNOSIS OF ABRUPTIO PLACENTAE
Test What It Measures Normal Value Value in Abruption
Bleeding time Vascular integrity and platelet 1-5 min Usually normal; test is of little clinical use in
function diagnosing abruption
Whole blood clotting time Platelet function Clot formation: 4-8 min Clot formation abnormality indicates severe
Fibrinolytic activity Clot retraction: <1 hr deficiency
Clot lysis: none in 24 hr Abnormal retraction with thrombocytopenia
Fibrinogen Fibrinogen level 400-650 mg/100 mL Usually decreased
Platelet count Number of platelets >140,000/mm3 Usually decreased
Fibrin degradation products Fibrin and fibrinogen <10 μg/mL Almost always elevated; most sensitive test
degradation products
Euglobulin clot lysis time Fibrinolytic activity None in 2 hr Difficult to interpret with low fibrinogen levels
Prothrombin time Factors II, V, VII, X 10-12 sec Normal to prolonged
Partial thromboplastin time Factors II, V, XIII, IX, X, XI 24-38 sec Normal to prolonged
Thrombin time Factors I, II 16-20 sec Parallels fall in fibrinogen; good marker of
Circulating split products abruption severity
Heparin effect
Red blood cell morphology Microangiopathic hemolysis Absence of distortion Presence of distortion or fragmentation is
or fragmentation uncommon but indicates risk of renal
cortical necrosis
TABLE 37-4 BLOOD REPLACEMENT PRODUCTS
Component Volume per Unit (mL)* Factors Present Effect of 1 Unit
Fresh whole blood 500 RBCs; all procoagulants ↑ Hematocrit 3%
Packed RBCs 200 RBCs only ↑ Hematocrit 3%
Fresh-frozen plasma 200-400 All procoagulants; no platelets ↑ Fibrinogen 25 mg/dL
Cryoprecipitate 20-50 Fibrinogen; factors VIII, XIII ↑ Fibrinogen 15-25%
Platelet concentrate 35-60 Platelets; small amounts of fibrinogen; ↑ Platelet count approximately 8000/mm3
factors V, VIII
RBCs, red blood cells.
*Volume depends on individual blood bank.

10. 734 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa
21. Lauria MR, Smith RS, Treadwell MC, et al: The use of second-trimester
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