Phases of the Menstrual Cycle

Ovaries contain approximately 400,000 follicles at birth; however, only 0.1% of them (400 or so) mature to ovulation

Days 0-14 of menstrual cycle:

The ovaries are in the follicular phase and the uterus in the proliferative phase. In the ovarian follicular phase, follicle stimulating hormone induces multiple follicles to begin development. By day 5-6 a dominant follicle (the one which will contains the egg to be released) emerges and the rest regress.

During this time the uterus is in the proliferative phase (estrogen induces growth (proliferation) of the functional layer of the endometrium to enable it to grow from about 1-3mm post cycle to about 6-8mm in thickness by ovulation). Ovulation (ejection of the mature egg from the ovary) and pick-up by the fallopian tube occurs generally between days 12-16 of the cycle.

Days 12-16 of the menstrual cycle:

Ovulation occurs when the egg from a dominant follicle (measuring 1.8-2.5 cm via ultrasound) is ejected from the ovaries and begins making its way down the fallopian tubes into the uterus. The endometrial layer is generally 6-8mm thick by this point in a cycle uninfluenced by “fertility drugs”.

Days 14-28 of the menstrual cycle:

The ovaries are in the luteal phase and the uterus is in the secretory phase in the ovarian luteal phase. Luteinizing hormone (LH) induces the formation of a corpus luteum (which degenerates if pregnancy does not occur). In the uterine secretory phase under the influence of estrogen and progesterone, the endometrium continues to grow. The glands become enlarged and if implantation does not occur the endometrium is shed after 28 days. Generally by day 28 of the cycle the endometrium measures between 10-16mm.

Day 1:

Menstruation occurs and the cycle begins again. It takes generally 5-7 days to shed the almost 1 cm of endometrium.

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