Menstruation and the Menstrual Cycle

Menstruation is the monthly vaginal discharge of blood and endometrial tissues that are shed from the lining of a woman’s uterus (womb). Also referred to as a period, menstruation is part of the menstrual cycle which prepares a woman’s body for pregnancy each month. It is an important indicator of general and reproductive health, although other health factors may limit this capacity.

The menstrual cycle for a woman has been compared with the moon, because the typical menstrual cycle mimics the lunar cycle of 28 days. The length of the cycle varies from woman to woman, and in some women it varies considerably from month to month as well depending on a variety of factors, including the woman’s physical, emotional, and nutritional health.

Cycles can range anywhere from 21 to 35 days in adults and from 21 to 45 days in young teens. This is because of the fact that women’s cycle tend to shorten and become more regular with age. However, the average cycle is 28 days long, counting from day 1 of a menstrual period up to (but no including) day 1 of the next period. While most periods last from 3 to 5 days, anywhere from 2 to 7 days is normal.

Phases of the Menstrual Cycle

The menstrual cycle can be thought of as consisting of two phases, the follicular phase and the luteal phase, with cyclicity occurring in a number of organs and tissues. The follicular phase (also called the preovulatory or proliferative phase), occurs from day 1 to 13; while the luteal phase (also called postovulatory or secretory phase), from days 14 to 28.

Between the follicular and luteal phases, on or about day 13 or 14 (at mid-cycle), ovulation – the release of an egg from the ovary – takes place. Ovulation is the shortest part of the menstrual cycle, lasting only a day or so. The ripened egg (ovum) bursts through the sac (follicle) that contains it and travels into the fallopian tubes which have become active at this point. As soon as the egg is released, the menstrual cycle enters its last phase.

Follicular Phase

The follicular phase is the first half of the monthly cycle, and it includes the days of menstruation. Due to the low levels of the hormones, estrogen and progesterone, toward the end of the previous cycle’s luteal phase, the hypothalamus is stimulated to produce gonadotropin-releasing hormone (GnRH). This in turn triggers the pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinising hormone (LH). FSH stimulates the growth of several follicles (each surrounding one egg) in the ovary, one of which will become the Graafian follicle, that is, the one activated to produce a mature egg cell. LH stimulates estrogen secretion by the follicles.

During this phase the levels of estrogen increases and also thereafter, LH suppresses the activity of all of the follicles but one – the Graafian follicle, which continues to develop and is groomed to complete maturity. Meanwhile, the endometrium thickens in preparation for the implantation of a fertilised egg.

As estrogen concentrations rise, FSH levels decline because, it is believed, the higher estrogen level now has a negative effect on the hypothalamus. LH levels remain constant, however. At mid-cycle, the end of the follicular phase, the high level of ovarian estrogen causes the pituitary to release a large amount of LH, which in turn stimulates the Graafian follicle to reach maturity and release an egg (ovulation); the empty follicle is then transformed into the corpus luteum, which secretes both progesterone and estrogen.

Luteal Phase

With ovulation, the follicular phase, that has been dominated by the hormone estrogen, ends. This is followed by the luteal phase which is to be dominated by the hormone progesterone, secreted (along with estrogen) by the empty follicle in its new form, the corpus luteum.

During this phase, the endometrium thickens more and the uterine glands begin to produce their secretions of life-supporting substances for the nurture of a fertilised egg. If the egg becomes fertilised by sperm and attaches to the uterine lining, the corpus luteum secretion now becomes controlled by a different hormone, human chorionic gonadotropin (HCG), manufactured by the placenta. If the egg is not fertilised, the secretions of the corpus luteum are still controlled by LH.

The higher levels of progesterone and estrogen produced by the corpus luteum now exert negative feedback (an inhibiting influence) on the production of LH and FSH, which begins to decrease. When the levels of LH drop, the corpus luteum begins to atrophy and stops producing estrogen and progesterone. The thickening of the endometrium can no longer be maintained and is therefore shed, along with some blood and mucus, in the form of the menstrual flow.

Very soon, however, the lowered hormone levels alert the hypothalamus to remedy the situation by producing LRH (luteinizing-releasing hormone) and FRH (follicle-releasing hormone), which in turn stimulate the pituitary production of LH and FSH, and the entire cycle begins again.

Tags: menstruation