During each menstrual cycle, along with ovarian changes, uterine changes also occur simultaneously.

Uterine changes occur in three phases:

1. Menstrual phase

2. Proliferative phase

3. Secretory phase.


After ovulation, if pregnancy does not occur, the thickened endometrium is shed or desquamated. This desquamated endometrium is expelled out through vagina along with blood and tissue fluid. The process of shedding and exit of uterine lining along with blood and fluid is called menstruation or menstrual bleeding. It lasts for about 4 to 5 days. This period is called menstrual phase or menstrual period. It is also called menses, emmenia or catamenia. The day when bleeding starts is considered as the first day of the menstrual cycle. Two days before the onset of bleeding, that is on 26th or 27th day of the previous cycle, there is a sudden reduction in the release of estrogen and progesterone from ovary. Decreased level of these two hormones is responsible for menstruation.



Changes in Endometrium during

Menstrual Phase

i. Lack of estrogen and progesterone causes sudden involution of endometrium

ii. It leads to reduction in the thickness of endometrium, up to 65% of original thickness

iii. During the next 24 hours, the tortuous blood vessels in the endometrium undergo severe constriction.

Endometrial vasoconstriction is because of three reasons:

a. Involution of endometrium

b. Actions of vasoconstrictor substances like prostaglandin, released from tissues of involuted endometrium

c. Sudden lack of estrogen and progesterone (which are vasodilators)

iv. Vasoconstriction leads to hypoxia, which results in necrosis of the endometrium

v. Necrosis causes rupture of blood vessels and oozing of blood

vi. Outer layer of the necrotic endometrium is separated and passes out along with blood

vii. This process is continued for about 24 to 36 hours viii. Within 48 hours after the reduction in the secretion of estrogen and progesterone, the superficial layers of endometrium are completely desquamated

ix. Desquamated tissues and the blood in the endometrial cavity initiate the contraction of uterus

x. Uterine contractions expel the blood along with desquamated uterine tissues to the exterior through

vagina. During normal menstruation, about 35 mL of blood along with 35 mL of serous fluid is expelled. The blood clots as soon as it oozes into the uterine cavity. Fibrinolysin causes lysis of clot in uterine cavity itself, so that the expelled menstrual fluid does not clot. However, in the pathological conditions involving uterus, the lysis of blood clot does not occur. So the menstrual fluid comes out with blood clot.

Menstruation stops between 3rd and 7th day of menstrual cycle. At the end of menstrual phase, the

thickness of endometrium is only about 1 mm. This is followed by proliferative phase.


Proliferative phase extends usually from 5th to 14th day of menstruation, i.e. between the day when menstruation stops and the day of ovulation. It corresponds to the follicular phase of ovarian cycle.

At the end of menstrual phase, only a thin layer (1 mm) of endometrium remains, as most of the endometrial stroma is desquamated.

Changes in Endometrium during

Proliferative Phase

i. Endometrial cells proliferate rapidly

ii. Epithelium reappears on the surface of endometrium within the first 4 to 7 days

iii. Uterine glands start developing within the endometrial stroma

iv. Blood vessels appear in the stroma

v. Proliferation of endometrial cells occurs continuously, so that the endometrium reaches the thickness of 3 to 4 mm at the end of proliferative phase. All these uterine changes during proliferative phase occur because of the influence of estrogen released from ovary. On 14th day, ovulation occurs under the

influence of LH. This is followed by secretory phase.


Secretory phase extends between 15th and 28th day of the menstrual cycle, i.e. between the day of ovulation and the day when menstruation of next cycle commences. After ovulation, corpus luteum is developed in the ovary. It secretes a large quantity of progesterone along with a small amount of estrogen. Estrogen causes further proliferation of cells in uterus, so that the endometrium becomes more thick. Progesterone causes further enlargement of endometrial stroma and further growth of glands.

Under the influence of progesterone, the endometrial glands commence their secretory function. Many

changes occur in the endometrium before commencing the secretory function.

Changes in Endometrium during

Secretory Phase

i. Endometrial glands become more tortuous. Because of increase in size, the glands become tortuous to

get accommodated within the endometrium

ii. Cytoplasm of stromal cells increases because of the deposition of glycogen and lipids

iii. Many new blood vessels appear within endometrial stroma. Blood vessels also become tortuous

iv. Blood supply to endometrium increases

v. Thickness of endometrium increases up to 6 mm. Actually, secretory phase is the preparatory period,

during which the uterus is prepared for implantation of ovum. All these uterine changes during secretory phase occur due to the influence of estrogen and progesterone. Estrogen is responsible for repair of damaged endometrium and growth of the glands. Progesterone is responsible for further growth of these structures and secretory activities in the endometrium. If a fertilized ovum is implanted during this phase

and if the implanted ovum starts developing into a fetus, then further changes occur in the uterus for the

survival of the developing fetus. If the implanted ovum is unfertilized or if pregnancy does not occur, menstruation occurs after this phase and a new cycle begins.

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