The Proliferative Phase : 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 oestrogen produced by the growing Graafian follicles in the ovary, the endometrium proliferates and begins to increase in size and number. New blood vessels grow from the stumps of the old. 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 muscle wall to the endometrial cavity.
In women who are undergoing infertility treatment, the endometrial thickness is measured by ultrasonography from one muscle wall of the uterus to the other – this measurement includes two layers of endometrium and the endometrial cavity in between, called the 'triple line endometrium'.
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 +1mm+3mm= 7 mm (approx)] by ultrasound. This is called the "triple line" of endometrium and indicates a good estrogenisation and healthy growth of the endometrium. A thickness of less than 5 mm is not favorable for the implantation of the embryo.
The Secretory Phase : This phase begins soon after ovulation occurs. Under the influence of progesterone produced by the corpus luteum in the ovary during its secretory phase, the endometium continues to grow to reach a maximum thickness of 5-7 mm as measured from the muscle wall to the endometrial cavity. The total measurement from muscle wall to muscle wall as seen on ultrasound during treatment for infertility will be about 10 mm.
In women with infertility undergoing ultrasonography, the total measurement of the two endometrial layers with the endometrial cavity in between will come to about 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 days. The length of the estrogenic or follicular phase (the time before ovulation) is what determines the length of the cycle, This is because the life of the ovum is for 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 days. So, when periods are delayed, it is due to delay of ovulation.
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.
Comparison between the Ovarian Cycle and the uterine Cycle :-
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