Applied Anatomy for the Obstetrician

1. Clinical Anatomy of
the Female Pelvis
For the Obstetrician
Professor Hassan Nasrat

2. The Bony Pelvis

5. The ileopectineal line
divides the pelvis into
the false and the true
pelvis
The normal female
pelvis is described as
“gynecoid” to be
differentiated from
the male “android
pelvis”.

7. The pelvic inlet “pelvic brim”
13
cm
Antero-posterior
Transverse
12
cm
Engagement of the fetal head usually occurs through the
transverse diameter

8. The Pelvic Cavity
The pelvic cavity:
Is the curved canal
between inlet and
outlet.
In the normal
female pelvis the
cavity is circular in
shape and curves
forwards.
All its diameters
measureapproximat
ely 12 cm.

10. The Pelvic Outlet
the two pubic bones make the pubic arch, which in the normal female pelvis forms an
angle not less than 90°. A narrow angle will force the fetal head at delivery posteriorly
and

11. Pelvic ligaments and
Diaphragm

12. The pudendal neurovascular bundle exits out of the greater sciatic foramen and
reenters the pelvis through the lesser sciatic foramen.
This is the site for administration of pudenal block for local anesthesia.

13. The two main muscles:
The levator ani muscle group:
Pubococcygeus, puborectalis, and iliococcygeus.
They muscles extend from the lateral pelvic walls
downward and medially to fuse with each other
posteriorly.
The levator hiatus lies anteriorly and accommodates the
urethra, vagina, and anus.
The coccygeus muscles
A triangular muscle arises from the ischial spine and
inserts onto the sacrum and coccyx

15. The Perineum

17. The Perineum
The perineum is divided into two parts (or
triangles):
Anterior or urogenital triangle:
Subdivided into:
A superficial and deep perineal spaces by a
fibromuscular septum called the urogenital
diaphragm
Posterior or anal triangle:
The midline attachment forms the fibromuscular
perineal body. between the anal canal and the
vagina

19. The Superficial Perineal Space
Boundaries of the
Superficial Perineal
Space
Note that the superfial muscles of the urogenital triangle and the muscles of
the anal triangle all converge in the midline at the central tendon of perineum
(perineal body) .

20. During episiotomy :It is important to
recognize superficial transverse perineil-muscle
in order to ensure proper cooptation.
Is bounded by three sets of muscles:
•The ischiocavernosus:
•The bulbocavernosus (the sphincter of the Vagina):
•The superficial transverse perinei:
It also includes the Bartholin’s glands and the
vestibular bulbs.
The superfial muscles of the urogenital triangle and
the muscles of the anal triangle all converge in the
midline.

21. The Deep Perineal Space

22. The Anal Triangle
The anal triangle is the area of the perineum behind an imaginary line
that extends between the ischial tuberosities.

23. The ischiorectal fossae :
•A potential space that allows distention of the
rectum during defecation and the vaginal wall during
second stage of labor.
•It is also a potential space for huge (up to one liter)
hematoma collection and abscess formation.
•The obturator nerve and internal pudendal
vessels: run alongside the lateral wall of the
ischiorectal fossa in the pudendal or Alcock’s canal.
This canal is formed from the splitting of the fascia
on the lateral wall of the ischiorectal fossa together
with the obturator fascia itself.

24. The external anal sphincter: The voluntary muscle which is responsible
for fecal continence is located within the anal triangle. Its total length is about 2 cm,
and it is composed
Tear of external anal sphincter is not uncommon during delivery particularly operative
one and should be carefully repaired. Failure to recognize tears of the external sphincter
or inappropriate repair can precipitate anal incontinence.

25. Nerve Supply of the Perineum
Ilioinguinal nerve (L1)
and genitofemoral
The Pudendal
nerve
(S2-4)
nerve (L1, 2)
Perineal branch of
posterior femoral
cutaneous nerve
Coccygeal and last sacral
nerves (S4, 5)

26. The Uterus
In 75% the uterus is in the anteverted, anteflexed position.
On rare occasion a retro-verted gravid uterus may get entrapped within the pelvis and
beneath the sacral promontory, giving rise to anterior sacculatoin of the uterus.
Clinically this presents with acute retention of urine.

27. The Isthmus is the short constricted area that marks the junction of the uterine
body with the cervix.

29. The body of the uterus:
It has three layers: The endometrium, the myometrium and the perimetrium:
The myometrium: Has longitudinal, circular and oblique muscle fibers
and is very expansile. The oblique muscle fibers run “criss-cross” and compress
the blood vessels when the uterus is well contracted.
It is found mostly in the upper segment of the uterus, where the placenta normally
embeds.
The richness in muscle fibers and its criss-cross important to ensure proper
hemostasis following placental delivery.
In contrast to that is the lower uterine segment which is poor hemostasis following
placental delivery.
This explains why bleeding in the third stage is more difficult to control if the
placenta is implanted in the lower uterine segment as in cases of placenta
praevia.
The Endometrium: During pregnancy and childbirth, the endometrium
is referred to as the decidua.
The perimetrium: Is a layer of peritoneum that covers the uterus except
at the sides where It extends to form the broad ligaments.
Significant bleeding and hematoma can extend whithin the layers of the broad
ligament into the extra peritoneal space with serious consequences

30. The Cervix:
Consists predominantly of collagenous connective tissue and
mucopolysaccaride ground substance.
It communicates with the uterine cavity through the internal os
and with the vaginal canal through the external os.
The endocervical canal is about 2.5 to 3 cm in length. It is lined
by a single layer of specialized columnar epithelium and
secretes mucus to facilitate sperm transport.
During pregnancy the glands secretion forms a plug of mucus
which helps protect against infection.
This plug of mucous comes away stained with some blood just
before labor commences. Many women refer to this as the
“show”.

31. Vascular Supply of the
pelvis

32. Note the anastomsis
between the ovarian and
uterine artery.
Therefore the uterus receive
blood supply from two
sources on each side
Note the Ureter Crosses
below the Uterine Artery
about 1 cm from the
cervix

34. Vessels and nerves of the
deep perineal space

35. Innervations of the
Pelvis Routes of Nerve Supply to
the uterus (visceral nerves).
Pain of uterine contractions
in the first stages is felt in
the abdomen, lower back
Routes of Nerve Supply to
cervix and upper vagina
(Somatic nerves)
In the second stage
additional source of pain
from cervical stretching and
perineal pressure.