11.70: Menstrual Cycle

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What happens during the menstrual cycle?

When you think of the menstrual cycle, you probably think of the discharge of blood that happens during menstruation. This is only one small part of the female monthly cycle, however.

The Female Monthly Cycle

The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month. While the egg and follicle are developing in the ovary, tissues are building up inside the uterus, the reproductive organ where the baby would develop. The uterus develops a thick lining covered in tiny blood vessels. This prepares the uterus to receive an egg that could develop into a child (a fertilized egg). The occurs during the first part of the cycle.

Ovulation, the release of an egg from the ovary, occurs at about the midpoint of the cycle. This would be around day 14 of a 28 day cycle. The egg is swept into the fallopian tube. If sperm is present, fertilization may occur. As sperm can only survive in the fallopian tube for up to a few days, fertilization can only occur within those few days post-ovulation.

If the egg is fertilized, the egg makes its way through the fallopian tube into the uterus, where it imbeds into the thick lining. When this occurs, the monthly cycle stops. The monthly cycle does not resume until the pregnancy is over.

If a sperm does not enter an egg, the lining of the uterus breaks down. Blood and other tissues from the lining break off from the uterus. They pass through the vagina and out of the body. This is called menstruation. Menstruation is also called a menstrual period. It lasts about 4 days, on average. When the menstrual period ends, the cycle repeats. Some women feel discomfort during this process.

Some people think that the average length of a menstrual period is the same as the “normal” length. They assume that shorter or longer menstrual periods are not normal. In fact, menstrual periods can vary from 1 to 8 days in length. This is usually normal. The average length of the cycle (time between menstrual periods) is about 28 days, but there is no “normal” cycle length.

Summary

The menstrual cycle is a monthly cycle of changes in the ovaries and uterus.
During menstruation, blood and other tissues from the lining of the uterus are shed and exit the body.

Review

What happens in the uterus as the egg and follicle are developing in the ovary? Why is this necessary?
What happens to the uterus if the egg is not fertilized?
Explain what happens during menstruation.