The Menstruation cycle for most women varies between 27 and 32 days. If it’s regular, you and your partner are not using any method of birth control and are having sex at least twice a week, there’s an 85 percent chance that you’ll get pregnant within a year. If pregnancy does not occur, then you and/or your partner may have a fertility problem. On the first day of your menstrual period, the hypothalamus (a small area in the brain behind the eyes) detects that the level of the female hormone estrogen is low.
It then sends a chemical signal to the pituitary gland (a pea–sized structure that hangs from the base of the brain) that it’s time to stimulate the ovaries to produce an egg. The pituitary reacts by sending a fertility hormone called follicle–stimulating hormone (FSH) into your bloodstream. One of your two ovaries responds by summoning anywhere from eight to 15 eggs from your total available lifetime supply of 40,000 (reduced at adolescence from approximately 2 million in the ovaries of a newborn––the fact that all the woman’s eggs are present from birth contributes to an increased risk of genetic abnormalities after age 35.) For unknown reasons, although suspected to be hormonal, only one of these eggs (or two in the case of fraternal twins) ripens completely each month, while the rest fail to mature and ultimately die. During the first 2 weeks of your Menstruation cycle, spurred on by FSH, the dominant egg develops and forms a blister–like egg sac known as a follicle near the surface of the ovary.
When the egg is almost ripe enough to be fertilized, FSH tells the ovary to release a surge of estrogen, which in turn signals the pituitary gland to produce large amounts of a second fertility hormone––Lutenizing Hormone (LH). Within 24 to 48 hours, the follicle releases a mature egg in response to LH and it makes its way to one of your two fallopian tubes. This is the big moment which you’ve been waiting for–Ovulation, when your body is ready to become pregnant. It’s here in the fallopian tube that the egg meets one of the approximately 200 to 600 millions of sperm from a single ejaculation and fertilization takes place. In the next 3 days, the fertilized egg travels down the tube to the uterus where it implants on day 5 to 6, in the uterine lining, which has been prepared to receive it during the first half of the menstrual cycle. If the egg isn’t fertilized, the lining sloughs resulting in the menstrual period.