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How the Menstrual Cycle Works

Most women learn in health class that every month their ovaries will release an egg, which will travel down a fallopian tube. A fertilized egg will most likely result in pregnancy, while an unfertilized one will lead to your period, called menstruation or menses. While this sounds simple, your reproductive system is actually an intricate organism and there is a lot that goes on inside of you before that egg can be released.

The Average Cycle
As the female fetus develops in the womb, she not only forms her ovaries, but also all the eggs she will ever produce. Shortly before birth, a female fetus has a few million immature eggs stored in her ovaries. Yet, by the onset of puberty, this number has already dwindled down to about 400,000. During the menstrual years, a woman will only release about 300 eggs. While each menstrual cycle usually sees the ovaries releasing one egg, what many women may not realize, though, is that their body actually prepares about 20 immature eggs every month for release. However, only one egg will prove to be dominant and this is the one that gets released for ovulation.

A woman's reproductive years begin in puberty when she starts menstruating and ends when she enters menopause, usually between the age of 45 and 50. While the average length of a menstrual cycle is 28 days, it is perfectly normal for a cycle to last anywhere from 21 to 35 days. On average, most women have menstrual flow for three to five days but it is very normal to menstruate for as much as seven days.

Hormones
There are five main hormones involved in the menstrual cycle process: estrogen, progesterone, gonadotropin releasing hormone (GnRH), follicle stimulating hormone (FSH) and lutenizing hormone (LH).

Estrogen: Although estrogen is thought of as one type of hormone, there are actually different types of estrogen produced by the female body. The two main estrogens involved in your menstrual cycle are estradiol and androgen. Estradiol is responsible for thickening the endometrial lining along with making your vaginal and cervical mucus more inviting for sperm. Androgen, on the other hand, does not actually start out as estrogen. Rather, the ovaries convert androgen to extra estrogen. This increase in estrogen helps to get rid of the immature egg follicles

Progesterone: This hormone is produced by the follicle from which the mature egg has been released (once the egg is released, this follicle is known as the corpus luteum). Progesterone helps make the endometrial lining ready for implantation if an egg is fertilized during the cycle. Progesterone also prevents the egg follicles from developing any further.

GnRH: Produced by the hypothalamus in the brain, GnRH is the controlling force behind the production and levels of estrogen in the body. Towards the end of your cycle, your estrogen levels bottom out and the GnRH is notified to start the production all over again. Once your body starts secreting high levels of progesterone, though, GnRH is no longer produced.

FSH: Just as the name suggests, increased levels of this hormone help to stimulate egg follicles, thereby aiding the maturation of the eggs as well as increasing the production estradiol. FSH is secreted by the pituitary gland, which is stimulated by the hypothalamus' production of GnRH.

LH: Also produced by the pituitary gland in response to your body's production of GnRH. This hormone works with the egg follicles to produce androgen.

The First Weeks
The first day of your menstrual cycle is the first day of your period. At this time, your estrogen levels are at their lowest which sends a signal to your brain that more FSH is needed. As your body produces more FSH, about 20 egg follicles begin to mature. However, only one of these egg follicles will attract the most FSH and begin to produce estradiol. Once estradiol begins to be secreted, your pituitary gland receives the signal to stop producing FSH in such high volumes. As the production of FSH tapers off, the weaker egg follicles die off.

With the increase in estrogen production, your uterine lining begins to thicken while your cervical mucus changes from thick and unwelcoming to sperm to thin and slippery, perfect for sperm to swim in. Not only does the estrogen change the consistency of your cervical mucus, but it also increases the amount of it, which will also help sperm get to your egg.

Ovulation
Since the beginning of your menstrual cycle, your estrogen levels have been steadily increasing. Approximately 36 hours before you ovulate, these levels will peak and cause the pituitary to release a surge of lutenizing hormone. When the follicle releases the egg, the fimbraie of the fallopian tubes (20 to 25 finger-like structures on the end of the fallopian tubes) collect the egg into the tube where it begins traveling down towards the uterus. This is known as ovulation.

Once ovulation occurs, the follicle from which the egg was released shrinks and begins to gather lipids, a type of fatty substance. This follicle then becomes known as the corpus luteum and begins to produce progesterone. As the corpus luteum secretes more and more progesterone, your uterine lining begins to get ready for implantation by a fertilized egg.

Once released, the egg is thought to have a 24-hour window during which time it can be fertilized by sperm. However, sperm can survive within the female body anywhere from three to seven days. Women who are looking to avoid a pregnancy should not have sex or should use a reliable form of birth control in the week leading up to ovulation.

Menstruation
If an egg is fertilized and successfully implants into your uterine lining during this cycle, your body will begin to secrete human Chorionic Gonadotropin (hCG). This hormone helps to continue the production of estrogen and progesterone for the remainder of your pregnancy. The corpus luteum will also continue to produce progesterone as it is stimulated by the fetus' placenta.

If an egg is not fertilized, it will be expelled along with the endometrial lining from your body. This is your period. Menstruation will also occur if an egg has been fertilized but fails to implant in the uterine lining. In this case, the fertilized egg along with the endometrium will be expelled during your period. Additionally, when no pregnancy occurs during a cycle, the corpus luteum dries up, becoming scar tissue, and your hormonal levels begin to fall. As they drop, the hypothalamus will be stimulated and start producing GnRH. This will signal the pituitary gland to start secreting FSH -- and so your menstrual cycle begins again.