In the beginning of the menstrual cycle, levels of estrogen rise, causing the lining of the uterus to grow and get thicker.  An egg starts to mature in one of the ovaries.  Around the middle of the cycle, the egg leaves the ovary, a process called ovulation.

The egg begins to travel down the fallopian tubes to the uterus.  If the egg becomes fertilized by a sperm cell and attaches to the uterus, the woman becomes pregnant.  If not, the uterus does not need the extra thick lining and it begins to shed.

This shedding of the uterine lining through the vagina is menstruation.

Most menstrual periods last from three to five days.  In the United States, most girls start menstruating at age 12, but girls can start menstruating between the ages of 8 and 16.

Some women have other symptoms around the time of menstruation, including:

• Cramping, bloating, and sore breasts
• Food cravings
• Mood swings and irritability
• Headache and fatigue

If these symptoms are severe, it might be a sign of premenstrual syndrome (PMS).  PMS usually occurs one or two weeks before menstruation.   PMS may affect a woman of any age who has menstrual periods. If the symptoms disrupt your lifestyle, you may want to seek medical care.