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 1 of the cycle is the first day of menstrual bleeding. Bleeding usually lasts for 4 to 6 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 7 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.