The menstrual cycle is the body's monthly pattern of preparing for a possible pregnancy. The lining of the uterus (endometrium) starts to thicken. One of the ovaries releases an egg. And then, if the egg isn't fertilized or a fertilized egg doesn't attach to the lining of the uterus (implant), 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. 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). Ovulation days can range from about day 7 to day 22 of the cycle. But for each person, the ovulation day is often on the same day each cycle.

• 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 a fertilized egg does not implant, the lining of the uterus is shed during the menstrual period. Then the cycle starts again.