Hesperian Health Guides

Women’s Menstrual Cycles

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About once each month during her reproductive years, a woman has a few days when a bloody fluid leaves her womb and passes through her vagina and out of her body. This normal monthly bleeding is called menstruation, or a menstrual period. Because the same pattern happens each month, it is called the menstrual cycle. Most women bleed every 28 days. But some bleed as often as every 20 days or as seldom as every 45 days.

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A woman’s ovaries release an egg once a month. If it is fertilized she may become pregnant. If not, her monthly bleeding will happen.

Menstruation is a normal part of women’s lives. Knowing how the menstrual cycle affects the body and the ways menstruation changes over a woman’s lifetime can let you know when you are pregnant, and help you detect and prevent health problems. Also, many family planning methods work best when women and men know more about the menstrual cycle (see Family Planning).

Hormones and the menstrual cycle

In women, the hormones estrogen and progesterone are produced mostly in the ovaries, and the amount of each one changes throughout the monthly cycle. During the first half of the cycle, the ovaries make mostly estrogen, which causes the lining of the womb to thicken with blood and tissue. The body makes the lining so a baby would have a soft nest to grow in if the woman became pregnant that month. When the soft lining is ready, an egg is released from one of the ovaries. This is called ovulation. The egg then travels down a tube into the womb. At this time a woman is fertile and she can become pregnant. If the woman has had sex recently, the man’s sperm may join with her egg. This is called fertilization.

During the last 14 days of the cycle, a woman also produces progesterone. Progesterone causes the lining of the womb to prepare for pregnancy. Most months, the egg is not fertilized, so the lining inside the womb is not needed. Then, the ovaries stop producing estrogen and progesterone, and the lining begins to break down. The monthly bleeding is when the lining inside the womb leaves the body. This is the start of a new monthly cycle. After the monthly bleeding, the ovaries start to make more estrogen again, and another lining begins to grow.

The monthly cycle
four drawings that show the changes in a woman's reproductive parts during the monthly cycle
During monthly bleeding, the lining breaks down.
Just after monthly bleeding
lining of the womb
When the ovary
releases an egg
...the lining
5 days after
ovulation, the
lining becomes
even thicker.
For most women, the whole menstrual cycle takes about 28 days —just like the cycle of the moon.

This page was updated:05 Jan 2024