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 first period is called menarche and is often a developmental defining moment for young girls marked by pride mingled with some anxiety. In the United States, the average age for the onset of menstruation is 12 years old. However, menstruation can be as early as 9 or 10 or as late as 16 or 17 for some girls, depending on a variety of factors which includes the young woman’s general health, nutritional status, and body weight relative to height. Usually the first period starts about 2 years after breasts first start to grow. Therefore, if a girl has not had her first period by age 15, or if it has been more than 2 to 3 years since breast growth started, she should see a doctor.

The Menstrual Cycle

Although termed the ‘menstrual cycle’, since menstruation is the obvious monthly event during reproductive life, the normal menstrual cycle is mostly a reflection of ovarian events. Monthly menstruation is an obvious marker that the various levels of interaction between hypothalamus, anterior pituitary gland, ovary and the uterus are functional. Thus the core events of the normal menstrual cycle are an integration of an interconnected neuroendocrine feedback loop: the hypothalamic-pituitary-ovarian axis.

The process is regulated by the levels and interactions of certain hormones within this loop, which govern the release of an egg from the ovary, the preparation of the endometrium (uterine lining) for pregnancy, and the shedding of the endometrial tissue (extra uterine tissues) when no pregnancy is begun. The uterus is not a core player in the normal menstrual cycle, but rather is a target organ for hormonal action. Any interruption of this axis at any point may lead to disordered menstruation.

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 its circle 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 fertilized 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 fertilized egg. If the egg becomes fertilized by sperm and then attach itself 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 fertilized, 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.