what causes menstruation

Okay, so I know that most people reading this have seen the movies and the film strips in junior high and have taken the mandatory biology courses, of which human reproduction is surely explained again, but consider this a refresher course. Lets start at the beginning:

When does it start:

Not every girl begins menstruating at the same age. Women, think back to your own experiences. Remember your friend Jennifer who "started" at age nine, you at 13 and Alice, lucky gal, who waited until you all were 16.

Why such a range of ages? The experts are not quite sure. At the average age of twelve, the hypothalamus begins releasing hormones that trigger the ovaries to get into gear and produce an egg. What causes the release is the question. One theory is that fat content is what triggers it all. Natalie Angier puts forth the "rule of thumb... that when a girl reaches approximately one hundred pounds, she pubesces, regardless of height or even her age" (178-179).


How does it progress:

Day one of the menstrual/ovulatory cycle is usually counted as the first day of menstruation, or the shedding of the uterine lining (Bio, 926). For the ovaries it is a quiet time, where relatively no hormones are released (Angier, 180).

Now begins the beginning of ovulation and what is known as "the period". The menstrual flow occurs for an average of five days during this time (Bio, 926). The lack of hormonal output from the ovaries signal hormonal output from the pituitary gland. It signals the follicles in the ovaries where eggs are "born" to get started. About twenty of them begin, but by day ten, on average, only one of them remains. The body allows only one follicle's egg to continue maturing, usually. The natural occurrence of multiple births is the result of more than one egg maturing past this stage (Angier 180-181).

Now begins the end of ovulation. The "liberation of the egg" occurs on a hormonal release from the pituitary gland. The follicle erupts and the egg is released into the waiting fallopian tube, where it begins its journey to the well-lined uterus. According to Angier, this can happen "on day twelve or fourteen or thereabouts" (181). It also marks the end of the proliferative phase, which is begins at the end of menstrual flow. This is the stage in which the endometrium, or lining in the uterus, grows thick as its arteries enlarge to provide nourishment for possible fertilized egg (Bio, 926).

Now, as the egg travels through the fallopian tube, waiting to be fertilized, the follicle it erupted from begins to undergo a change. It develops the corpus luteum, or "yellow body". The corpus luteum is responsible for furthering the thickening of the endometrium and preparing the rest of the body for a possible pregnancy. If conception does not occur, the corpus luteum disintegrates and signals the endometrium. If pregnancy does occur, the presence of the corpus luteum keeps the cycle from starting again (Angier, 182-183). This is the secretory phase of menstruation, where the endometrium continues to grow. It lasts approximately two weeks and ends with the shedding of the top layers of the endometrium, or menstrual flow: back to day one (Bio, 926).


When does it end:

The menstrual cycle ends at a time known as menopause. The relief, or horror, of "The Change." It occurs, on average, between the ages of forty-six and fifty-four. It is started by the ovaries lack of responsiveness to the hormonal signals produced by the pituitary gland (Bio, 927).

What effects changes in the norm:

While the menstrual cycle cannot be controlled by any form of mind over matter, there are events that could occur to stop or delay its timing. The most variable of times in the cycle is that of ovulation. It can be delayed due to physical stress, such as malnutrition or the flu. Angier puts this phenomenon into logical terms, "if you're seriously ill, you need to focus on getting well. You can't afford to divert energy to a pregnancy" (183-184).

One very intriguing, and widely debated idea, is that of menstrual synchrony, or the idea that women in close contact with one another actually have a shift in their cycles so that the two occur simultaneously. The original study published on this occurrence first came out in the journal Nature in 1971. It claimed that menstrual synchrony did occur. According to Angier, later studies confirm the first, some refute the results and one even proves the opposite (186). As you can see, it is still a bit of an unknown arena.

One study, though, goes a bit further than simply trying to observe the effect: it attempts to find the cause. Martha McClintock, author of the first study, published results in Nature in 1998 from an experiment her team undertook. Angier summarizes the results nicely:

'The scientists showed that if they took swabs from the armpits of women at different points in their ovulatory cycle and applied the swabs to the upper lips of other women, the donor secretions could act as pheromones, as odorless chemical signals. The secretions either hastened or prolonged the cycles of many, though not all, of the women exposed to them (188).'

Angier claims that the though not all women were affected, enough were to prove statistical significance. She also claims that this result, though not completely understood, would account for the widely varying results of other menstrual synchrony studies (189).


Birth Control:

Of course, women taking any form of the birth control pill in the prescribed manner, are regulating themselves. They are ingesting the hormones that their bodies would be producing in order to "fool" themselves out of actual pregnancies. Any variation in their administration of the pill would cause a change in their cycle, but if no variations occur, the are "unnaturally" regular (Planned Parenthood Pamphlet).