The menstrual cycle is a term used to describe the sequence of events that occur within a woman’s body as it prepares for the possibility of pregnancy each month. A menstrual cycle is considered to begin on the first day of a period. The average cycle is 28 days long; however, a cycle can range in length from 21 days to about 35 days.
The steps in the menstrual cycle are triggered by the rise and fall of chemicals in the body called hormones. The pituitary gland in the brain and the ovaries in the female reproductive tract manufacture and release certain hormones at certain times during the menstrual cycle that cause the organs of the reproductive tract to respond in certain ways. The specific events that occur during the menstrual cycle can be described as follows:
1. The menses phase. This phase, which typically lasts from day 1 to day 5, is the time when the lining of the uterus is actually shed out through the vagina if pregnancy has not occurred. Most women bleed for 3 to 5 days, but a period lasting only 2 days to as many as 7 days is still considered normal.
2. The follicular phase. This phase typically takes place from days 6 to 14. During this time, the level of the hormone estrogen rises, which causes the lining of the uterus (called the endometrium) to grow and thicken. In addition, another hormone—follicle-stimulating hormone—causes follicles in the ovaries to grow. During days 10 to 14, one of the developing follicles will form a fully mature egg (ovum).
3. Ovulation. This phase occurs roughly at about day 14 in a 28-day menstrual cycle. A sudden increase in another hormone—luteinizing hormone—causes the ovary to release its egg. This event is called ovulation.
4. The luteal phase. This phase lasts from about day 15 to day 28. After the egg is released from the ovary it begins to travel through the fallopian tubes to the uterus. The level of the hormone progesterone rises to help prepare the uterine lining for pregnancy. If the egg becomes fertilized by a sperm and attaches itself to the uterine wall, the woman becomes pregnant. If pregnancy does not occur, estrogen and progesterone levels drop and the thickened lining of the uterus is shed during the menstrual period.