The menstrual cycle is the series of changes your body goes through to prepare for a possible pregnancy. About once a month, the uterus grows a new, thickened lining (endometrium) that can hold a fertilized egg. When there is no fertilized egg to start a pregnancy, the uterus then sheds its lining. This is the monthly menstrual bleeding (also called menstruation or menstrual period) that you have from your early teen years until your menstrual periods end around age 50 (menopause).

See a picture of a woman's reproductive system.

The menstrual cycle is measured from the first day of menstrual bleeding, Day 1, up to Day 1 of your next menstrual bleeding. Although 28 days is often cited as the "regular" cycle length, only 15% of women actually have such a cycle.⁵

• A teen's cycles tend to be long (up to 45 days), growing shorter over several years.²
• Between ages 25 and 35, most women's cycles are regular, generally lasting 21 to 35 days.¹
• Around ages 40 to 42, cycles tend to be the shortest and most regular. This is followed by 8 to 10 years of longer, less predictable cycles until menopause.⁵

Three phases of the menstrual cycle

The phases of your menstrual cycle are triggered by hormonal changes.

Menstrual period

On Day 1 of your cycle, the thickened lining (endometrium) of the uterus begins to shed. You know this as menstrual bleeding from the vagina. A normal menstrual period can last 4 to 6 days.⁵

Most of your menstrual blood loss happens during the first 3 days. This is also when you might have cramping pain in your pelvis, legs, and back. Cramps can range from mild to severe. The cramping is your uterus contracting, helping the endometrium shed. Generally, any premenstrual symptoms that you've felt before your period will go away during these first days of your cycle.

Follicular phase

During the follicular phase, an egg follicle on an ovary gets ready to release an egg. Usually, one egg is released per cycle. This process can be short or long and plays the biggest role in how long your cycle is. At the same time, the uterus starts growing a new endometrium to prepare for pregnancy.

The last 5 days of the follicular phase, plus ovulation day, are your fertile window. This is when you are most likely to become pregnant if you have sex without using birth control.

Luteal (premenstrual) phase

This phase starts on ovulation day, the day the egg is released from the egg follicle on the ovary. It can happen any time from Day 7 to Day 22 of a normal menstrual cycle. During ovulation, some women have less than a day of red spotting or lower pelvic pain or discomfort (mittelschmerz). These signs of ovulation are normal.

• If the egg is fertilized by sperm and then implants in (attaches to) the endometrium, a pregnancy begins. (This pregnancy is dated from Day 1 of this menstrual cycle.)
• If the egg is not fertilized or does not implant, the endometrium begins to break down.