Normal Menstrual Cycle

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

See a picture of a Reference woman's reproductive system Opens New Window Reference Opens New Window.

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 the average cycle length, it is normal to have a cycle that is shorter or longer.

• A teen's cycles may be long (up to 45 days), growing shorter over several years.^{Reference 1}
• Between ages 25 and 35, most women's cycles are regular, generally lasting 21 to 35 days.^{Reference 2}
• 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.^{Reference 3}

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 (Reference endometrium Opens New Window) 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.^{Reference 3}

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. In general, any Reference premenstrual symptoms Opens New Window 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 Reference ovary Opens New Window 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 Reference birth control Opens New Window.

Luteal (premenstrual) phase

This phase starts on Reference ovulation Opens New Window 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.

After the teen years and before Reference perimenopause Opens New Window in your 40s, your luteal phase is very predictable. It normally lasts 13 to 15 days, from ovulation until menstrual bleeding starts a new cycle. This 2-week period is also called the "premenstrual" period.

Many women have premenstrual symptoms during all or part of the luteal phase. You may feel tense, angry, or emotional. You may gain water weight and feel bloated. Or you may have tender breasts or acne. A day or more before your period, you may start to have pain (cramps) in your abdomen, back, or legs. It is normal to have less energy at this time. Some women also have headaches, diarrhea or constipation, nausea, dizziness, or fainting.

When premenstrual symptoms make your daily life difficult, you are said to have Reference premenstrual syndrome (PMS Opens New Window). For more information, see the topic Reference Premenstrual Syndrome (PMS).

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