THE UTERINE CYCLE
( Changes in the Uterus during the Menstrual Cycle )
Written by Dr.M.D.Mazumdar, MD
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 menstruation.
The Uterine cycle of menstruation starts from the first day of the menstrual period, which is also the first day of bleeding. It can be divided into three distinct phases:
This phase is also called the 'bleeding phase' or the 'menstrual period'. It usually lasts from the 1st to the 5th day of the cycle. The duration of bleeding can vary from 2 days to 7 days in different women. Usually the flow is less on the first day, increases during the 2nd and the 3rd day and peters off on the 4th and 5th days.
The menstrual discharge or the bleeding during he period consists of dead endometrial cells, blood, vaginal cells, mucous from the cervix and other necrotic tissue.
The main cause for the occurance of menstruation is the withdrawal of the ovarian hormones - oestrogen and progesterone - due to the degeneration of the corpus luteum in the ovaries at the end of the ovarian cycle.
The lack of hormones causes the endometrium to shrink. The blood vessels of this region become highly coiled on themselves and the blood stagnates in these coils. The endometrial cells get necrosed due to unavailability of blood. The endometrial tissue soon dies off and passes out of the uterus and vagina as menstrual bleeding.
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.
Under the influence of estrogen produced by the growing Graafian follicles in the ovary, the endometrium, mainly the endometrial cells, proliferates and begins to increase in size and thickness. New blood vessels grow from the stumps of the old vessels. The endometrial glands increase in size, though not in number. The stromal cells assume a compact arrangement. At the end of the proliferative stage, i.e. on the 14th day, the endometrium is 2-3 mm thick.
This is however, the measurement of the endometrium on one side of the uterine wall only - from the inner edge of the muscle wall to the endometrial cavity.
Since the endometrial cavity is about 1-2 mm in width, the total endometrial thickness at the time of ovulation measures about 6 - 8 mm [ 3mm thickness of the endometrium on one side + 1mm of the endometrial cavity + 3mm of the endometrium on the other side = 7 mm (approx)] by ultrasound. This is called the "triple line" of endometrium and indicates good estrogenisation and healthy growth of the endometrium. A thickness of less than 5 mm is not favorable for the implantation of the embryo.
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This phase begins soon after ovulation occurs. At this time, the endometrial thickness is about 7-8 mm as measured from one muscle wall to the other. Under the influence of progesterone produced by the corpus luteum in the ovary during its secretory phase, the total measurement from muscle wall to muscle wall as seen on ultrasound by the end of this phase is about 10 mm.
In women who receive stimulatory medicines to treat infertility, the total measurement by ultrasonography of the endometrial thickness may be as high as 10 - 15 mm. The stromal cells continue to increase in size and number. Blood supply to the endometrium increases.
The important change in this phase occurs in the endometrial glands. The glands increase in size and become actively secretory. Hence the name - 'secretory phase'. In the early stage, the secretions collect in the cells of the glands.
But by the 19th to the 22nd day of the cycle, the secretions are pushed out of the cells and collect in the endometrial cavity. This secretion is rich in glycogen, fructose and glucose. Its main function is to supply nutrition to any fertilized ovum reaching the uterus.
The luteal phase (or progesterogenic phase) is always of the same duration - 14-16 days. This is because the life of the ovum is only 14 days.
Once the ovum is released at ovulation, it can have only two options. If fertilized, it will develop into a new individual. If not fertilized, it will degenerate and die within 14-16 days. So, when periods are delayed, it is usually due to delay of ovulation. The length of the estrogenic or follicular phase (the time before ovulation) is what determines the length of the cycle.
Endometrial growth stops from the 22nd day of the cycle as the corpus luteum degenerates. Then it starts to shrink in the next few days. Necrosis occurs with shedding of the endometrial lining and bleeding. This indicates the beginning of the next menstrual cycle.
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The Menstrual Phase
The Proliferative Phase of Menstruation
In women who are undergoing infertility treatment, the endometrial thickness is measured by ultrasonography from inner edge of one muscle wall of the uterus to inner edge of the other - this measurement includes two layers of endometrium and the endometrial cavity in between, called the 'triple line endometrium'.
The Secretory Phase of Menstruation
