This document discusses the normal ultrasound appearance of the endometrium and ovaries throughout the menstrual cycle. It provides details on the expected endometrial thickness at different phases of the cycle. It also describes the sonographic signs of ovulation and the appearance and development of the corpus luteum. Post-menopausal endometrial thickness is also addressed. Failure of ovulation and the development of luteinized unruptured follicles are summarized at the end.

1. USG in normal menstrual cycle

2.  The changes in the internal uterine lining of
the uterus - the endometrium - during the
menstrual cycle is termed the uterine cycle of
menstruation.
 These changes occur in response to the
hormones, estrogen and progesterone,
secreted by the ovaries during the ovarian
cycle of menstruatio -

3.  Endometrial thickness is a commonly
measured parameter on
routine gynaecological ultrasound and MR
imaging.
 The appearance, as well as the thickness of
the endometrium, will depend on whether
the patient is of reproductive age or post-
menopausal and, if of reproductive age, at
what point in the menstrual cycle they are
examined.

4.  The endometrium should
be measured in the
long axis or sagittal plane.
 The measurement is of the
thickest echogenic area
from one basal
endometrial interface
across the endometrial
canal to the other basal
surface.
 Care should be taken not
to include the hypoechoic
myometrium in this
measurement.

5. Premenopausal
 In premenopausal patients,
there is significant variation at
different stages of the
menstrual cycle.
 during menstruation : 2-4 mm
 early proliferative phase (day 6-
14): 4-8 mm
 late proliferative-pre ovulatory
phase: up to 11 mm
 secretory phase: 7-16 mm
 following dilatation and
curettage or spontaneous
abortion: <5 mm, if it is thicker
consider retained products of
conception.

6.  At birth, the uterus is similar in
size to the cervix (2.3–4.6 cm),
and the endometrium generally
appears as a thin, echogenic line
 Approximately one-fourth of
neonates will have fluid
collections within the
endometrial cavity
 Once puberty is reached, the
appearance of the endometrium
begins to approximate that seen
in adulthood and varies with the
stage of the menstrual cycle.

7. Postmenopausal EM
 Will depend on the whether or
not there is a history of vaginal
bleeding, and on the use of
hormonal therapy / tamoxifen.
 Homogeneous, smooth
endometria measuring 5 mm
or less are considered within
the normal range with or
without hormonal
replacement therapy
 if on tamoxifen 3: <6 mm
(although ~50% of those
receiving tamoxifen have been
reported to have a thickness of
>8 mm 7)

8.  in a patient undergoing
hormonal replacement therapy
may vary up to 3 mm if cyclic
estrogen and progestin therapy
is being used
 The endometrium will appear
thickest prior to progestin
exposure and thinnest after the
progestin phase. Imaging should
be performed at the beginning
or end of a cycle of treatment,
when the endometrium will be
at its thinnest and any
pathologic thickening will be
most prominent.
 A patient undergoing
unopposed estrogen therapy
with endometrial thickening
exceeding 8 mm should be
considered for biopsy, whereas
patients receiving progesterone
in addition to estrogen can be
rescanned at the beginning or
end of the following cycle to
determine if there has been a
change in endometrial thickness
(22).

10.  During menstruation,
the endometrium
appears as a thin,
echogenic line 1–4 mm
in thickness

11.  Once the menstrual
bleeding stops there is a
short duration of about
48 hours when the
endometrium rests and
repairs itself ('resting
phase').
 At this time, the
endometrium is
disorganized and chaotic
and only about 1mm
thick.

12.  In the late proliferative
(periovulatory) phase, the
endometrium develops a
multilayered appearance
with an echogenic basal
layer and hypoechoic inner
functional layer, separated
by a thin echogenic
median layer arising from
the central interface or
luminal content

14.  in the secretory phase it
is at its thickest
and becomes uniformly
echogenic, as the
functional layer becomes
oedematous and
isoechoic to the basal
layer .
 There is thorough
transmission and
posterior acoustic
enhancement noted.

15.  Endometrial growth
stops from the 22nd day
of the cycle as the
corpus luteum
degenerates.Then it
starts to shrink and then
necrosis occurs with
shedding of the
endometrial lining and
bleeding.
 And thus starts the next
menstrual cycle.

17.  Journey to ovulation begins during late luteal
phase of prior menstrual cycle, when certain
2-5 mm sized healthy follicles form a
population, from which dominant follicles is
to be selected for next cycleThis process is
called 'recruitment'.
 Usual number of such follicles may be 3-11,
which goes on decreasing with advancing
age1

18.  During Day 1-5 of the
menstrual cycle, a
second process of
'follicular
selection' begins, when
among all recruited
follicles, certain growing
follicles of size 5-10 mm
are selected, while rest
of the follicles regress or
become atretic.

19.  During Day 5-7 of the menstrual
cycle, a process of 'dominance'
begins, when a certain follicle of 10
mm size takes the control and
becomes dominant.
 This also suppresses the growth of
the rest of the selected follicles, and
in a way, is destined to ovulate.
 This follicle starts growing at rate of
2-3 mm a day and reaches 17-27 mm
size just prior to ovulation
 One important learning point in this regard is,
"largest follicle on day 3 of the cycle, may or
may not be a dominant follicle in the end.
Process of dominance begins late, when
suddenly a certain underdog follicle starts
growing faster and suppresses others to
become dominant".

21.  Almost nearing ovulation, rapid
follicle growth takes place, and
follicle starts protruding from
the ovarian cortex, attains a
crenated border, and it literally
explodes to release the ovum,
along with some antral fluid.

22. Ovulation is determined by
following sonographic signs:
 follicle suddenly disappears
or regresses in size
 irregular margins
 intra-follicular echoes. Follicle
suddenly becomes more
echogenic
 free fluid in the pouch of
Douglas
 increased perifollicular blood
flow velocities, on doppler

23.  At ovulation the follicle ruptures
expelling the ovum into the fallopian
tube.
 The remnants of the follicle are called
the corpus luteum and ranges from 2-5
cm. As it matures, it involutes.
 The corpus luteum produces oestrogen
and progesterone, maintaining optimum
conditions for implantation if the ovum is
fertilised:
 if fertilised: the corpus luteum
continues to produce these hormones
and maximises the chance of
implantation into the endometrium; it
reaches a maximum size at ~10 weeks
and finally resolves at around 16-20
weeks
 not fertilised: the corpus luteum
involutes and turns into a corpus
albicans by around 2 weeks

24.  The corpus luteum is an endocrine
gland responsible for helping to
regulate the menstrual cycle and
support early pregnancy.
 Cells of the preovulatory follicle
wall contribute to the formation of
the corpus luteum by structural and
functional transformation that
begins just prior to follicle rupture.
 Perifollicular capillaries fenestrate
the basal lamina of the follicle wall,
the basal lamina breaks down and
luteal cells arise from theca interna
and granulosa cells.66
 Neoangiogenesis of the corpus
luteum facilitates its endocrine
gland activity.

25.  diffusely thick wall
 peripheral vascularity
 <3 cm
 possible crenulated contour
 If the cyst has been present for
some time with complicating
haemorrhage, a fine internal lace
like echo-pattern may be seen.
 Colour Doppler
interrogation show either no
vascularity within the cyst or at
times show low resistance blood
flow around the cyst also known
as hypervascular ''ring of fire''.

26.  Degradation of vascular flow
accompanies luteolysis, the
regression of the corpus luteum in
the late luteal phase of each
menstrual cycle, in the absence of
conception.
 Following luteal regression, the
corpus albicans may be visualized
until the time of subsequent
ovulation
 Corpus albicans are typically
visualized as hyperechoic structures
within the ovary and they may
occasionally appear to be more
pronounced owing to the presence
of surrounding follicles.

27.  Failure of ovulation and
development of “cystic” follicle.
 The follicle typically grows larger
than the mean preovulatory follicle
diameter of 23 mm, thin atretic
follicle walls are observed and small
flecks of particulate matter are
frequently seen in the lumen or
aggregated at the side of the
structure.
 Infertility can also be associated
with growth of a dominant follicle
beyond a preovulatory diameter
and subsequent formation of a large
anovulatory follicle cyst.
 No luteinization of the follicle wall
occurs and the follicle wall is thin
and displays marked hyperechocity
 The follicular fluid remains
clear/hypoechoic.

28.  Following release of the
preovulatory surge of LH, the
preovulatory-size dominant
follicle fails to rupture.
 This results in retention of the
oocyte/cumulus complex is
within the lumen of the LUF.
 The follicle wall thickens and
attains gray scale and vascular
features similar to luteal tissue
 There is also a hazy indistinct
border between the follicle
fluid and the follicle wall.

29.  In addition, the point of follicle rupture, a
characteristic that distinguishes the LUF from a cystic
corpus luteum, is absent.
 Typically the mid-luteal progesterone concentration
and basal body temperatures are lower than would be
anticipated following normal ovulation.
 Menstrual flow does occur but menses are often
lighter than usual.
 The mechanism for the formation of the LUF is
uncertain and may include an ill-timed or attenuated
release of the surge of LH or may be due to a defect in
the follicle that makes it unresponsive to a normal LH
surge such as aberrant or reduced receptors for LH.