The menstrual cycle is a natural process that occurs in people with female reproductive systems. It is a complex cycle controlled by female hormones that cause regular bleeding, known as periods. The menstrual cycle prepares the body for pregnancy. If pregnancy does not occur, hormones signal the uterus to shed its lining, which becomes the period. Once the period starts, the cycle begins again. The menstrual cycle has four phases:

1. Menstruation: This is the shedding of the lining of the uterus accompanied by bleeding. It occurs in approximately monthly cycles throughout a persons reproductive life, except during pregnancy. Menstrual bleeding usually lasts 4 to 8 days, and blood loss during a cycle usually ranges from 1/5 to 2 1/2 ounces.

2. Follicular phase: This phase occurs before the release of the egg. Luteinizing hormone and follicle-stimulating hormone, which are produced by the pituitary gland, promote ovulation and stimulate the ovaries to produce estrogen and progesterone.

3. Ovulatory phase: This is when the egg is released from the ovary.

4. Luteal phase: This phase occurs after the egg is released. Estrogen and progesterone stimulate the uterus and breasts to prepare for possible fertilization.

The menstrual cycle is measured from the first day of a persons period to the first day of their next period. The average length of a menstrual cycle is 28 to 29 days, but every persons cycle is different. A normal menstrual cycle is a cycle that occurs roughly every 21 to 35 days and causes bleeding for between three and seven days. The menstrual cycle can change from a persons teen years to their 40s or 50s. When someone first gets their period, its normal to have longer cycles or a heavier period flow. It can take up to three years for young people to have regular cycles after they begin menstruating.