Women's Health Tutorial

Menstrual Cycle Information

Menstruation is just one part of the menstrual cycle, in which a woman's body prepares for pregnancy each month. A cycle is counted from the first day of one period to the first day of the next. An average cycle is 28 days, but anywhere from 23 to 35 days is normal.

Estrogen and progesterone levels are very low at the beginning of the cycle. As your period ends, levels of estrogen, made by the ovaries, start to rise and make the lining of the uterus grow and thicken. In the meantime, an egg (ovum) in one of the ovaries starts to mature. It is encased in a sac called the Graafian follicle.

Ovulation usually occurs at about day 14, when the ovary releases the egg and it travels through one of the fallopian tubes to the uterus. Some women know when they are ovulating, because they have some pain (typically a dull ache on either side of the lower abdomen lasting a few hours) or very light bleeding or spotting. After the egg is expelled, the sac--now called a corpus luteum--remains in the ovary, where it starts producing progesterone. The rising levels of both estrogen and progesterone help build up and stablilize the uterine lining to prepare for pregnancy.

The few days before, during, and after ovulation are considered a woman's "fertile period"-the time when she can become pregnant. Because the length of menstrual cycles vary, many woman ovulate earlier or later than day 14. It's even possible for a woman to ovulate while she still has her period if that month's cycle is very short. Hence, there is no time of the month during which a woman can be sure she will not get pregnant after unprotected sex. Stress and other things can sometimes cause a cycle to be shorter or longer. If a woman has sex with a man during this time and conception occurs (his sperm fertilizes the egg), she becomes pregnant. The fertilized egg attaches to the uterus, and the corpus luteum makes all the progesterone needed to keep it implanted and growing until a placenta (an organ connecting the fetus to the mother) develops. The placenta then makes hormones and provides nourishment from the mother to the baby.

If an egg is not fertilized that month and the woman doesn't get pregnant, the corpus luteum stops making hormones and gets reabsorbed in the ovary. Hormone levels drop again, the lining of the uterus breaks down, menstruation begins, and the cycle repeats.

In the illustration below, an egg has left an ovary after ovulation and is on its way through a fallopian tube to the uterus.

Female reproductive system

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