What is the menstrual cycle?

The menstrual cycle is the female body's monthly pattern of preparing for a possible pregnancy. The uterus grows a new lining called the endometrium; one of the ovaries releases an egg; and then, if the egg is not fertilized by sperm, the endometrium sheds from the uterus as a menstrual period.

• Menstrual phase (period). The thickened lining of the uterus (endometrium) is shed, causing menstrual bleeding. Day one of the cycle is the first day of menstrual bleeding. Bleeding usually lasts for four to six days.
• Follicular phase. The lining of the uterus thickens to prepare for pregnancy. Also, inside a sac (follicle) on the surface of an ovary, an egg becomes ready to be released.
• Luteal phase. The egg is released (ovulation). Women's ovulation days can range from about day seven to day 22 of the cycle. But for an individual woman, the ovulation day is usually quite consistent.
• If the egg is fertilized by sperm, it may attach to (implant in) the lining of the uterus, and pregnancy begins.
• If the egg is not fertilized or does not implant, the lining of the uterus is shed during the menstrual period, and the cycle starts again.