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Reviewed by the Faculty of Harvard Medical School

How Your Menstrual Cycle Works

The menstrual cycle is an impressive series of events — it is complicated and consistent from one woman to another. Throughout each cycle, your ovaries and your brain "dialogue" with each other, communicating by way of hormones. If you understand how your menstrual cycle works, you can predict the timing of your ovulation. This can help to explain symptoms you may experience during and between your periods, and importantly, it can help you to be informed about your fertility. If you are hoping to become pregnant, you may even choose to schedule intercourse for the days when you are most likely to conceive. The cycle for most women varies between 27 and 32 days.

The first day of your menstrual period is also considered the first day of your month-long cycle. Your ovaries normally produce varying quantities of the female hormones estrogen and progesterone, but at the very start of your cycle your ovaries aren't very active. The hypothalamus (a small area in the brain behind the eyes) detects that the level of the estrogen is low.

The hypothalamus then sends a chemical signal to the pituitary gland (a pea-sized structure at the base of the brain). This signal causes release of a hormone that can stimulate the ovaries, causing the ovaries to begin to mature a follicle (an immature egg) into a mature egg. This fertility hormone is called follicle-stimulating hormone (FSH). FSH travels to the ovaries through your bloodstream.

In reality, during each cycle your ovaries respond to FSH by starting to enlarge anywhere from 8 to 15 immature eggs from your total available lifetime supply. The egg that happens to mature fastest (the "dominant follicle") begins to generate a variety of hormones of its own. The mixing of these hormones from the dominant follicle with the other hormones that are circulating may be the reason that all other follicles stop advancing. In any given menstrual cycle, only the dominant follicle (or two dominant follicles in the case of fraternal twins) matures completely and is released at ovulation, while the rest fail to mature and ultimately die.

The ovaries of a newborn have approximately 1 to 2 million eggs, but by puberty there only will be approximately 300,000 to 400,000 eggs. The fact that all eggs are present from birth contributes to an increased risk of genetic abnormalities for babies who are conceived after the mother reaches age 35, because the eggs have had more time during which their DNA may potentially be damaged. It is estimated that only 1% of the 300,000 to 400,000 will ovulate in the course of a woman's lifespan.

It takes the first two weeks of your cycle, spurred on by FSH, for the dominant egg to completely mature. It forms a blisterlike egg sac near the surface of the ovary. When the egg is almost fully matured, it manufactures a good deal of estrogen and other hormones, affecting the bloodstream hormone "mix." The mix of hormones that occurs when the egg has matured is uniquely able to signal the pituitary gland to abruptly release a large amount of its next fertility hormone, luteinizing hormone (LH).

This surge of LH appears to trigger the release of the egg by the ovary, called ovulation. Within 24 to 48 hours, the follicle releases its egg and the egg makes its way into one of the two fallopian tubes. Normal ovulation can be a painful event for some women. Pain at ovulation is called mittelschmirtz. Ovulation occurs in the middle of the menstrual cycle — usually 14 days before the next menstrual period is due. A newly ovulated egg can be fertilized by a sperm any time in the next several days.

Back in the ovary, the cells that line the empty egg sac (called the corpus luteum) do not last for longer than a few days before deteriorating, except if the departed egg is fertilized. In this event, a new hormone resulting from pregnancy (human chorionic gonadotropin) helps to preserve the health and continuing function of these cells for a while longer.

Throughout the time that the egg is maturing prior to ovulation, the uterine lining is thickening, nourished by circulating estrogen. In the several days after ovulation (and before the corpus luteum deteriorates), cells in the corpus luteum produce progesterone. Progesterone changes the thickened uterine lining so that it is more uniformly ready to be shed, as a period.

A menstrual period begins if there is no implanted egg, once the corpus luteum deteriorates and progesterone levels decline. On this day, the ovaries aren't very active. The hypothalamus, of course, detects that the level of estrogen is low. This begins the cycle anew.

Last updated September 16, 2011




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