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# 13.64: Menstrual Cycle

Difficulty Level: At Grade Created by: CK-12
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What's the most important part of the female menstrual cycle?

A menstrual cycle calendar. A lot of things to keep track of. And for a few very important reasons, it is important to know when a woman is ovulating. But what's the most important part of the female menstrual cycle? That depends on who you ask.

### Menstrual Cycle

Ovulation is part of the menstrual cycle, which typically occurs each month in a sexually mature female unless she is pregnant. Another part of the cycle is the monthly period, or menstruation. Menstruation is the process in which the endometrium of the uterus is shed from the body. The menstrual cycle is controlled by hormones from the hypothalamus, pituitary gland, and ovaries. For an interactive animation of the menstrual cycle, you can go this link: http://health.howstuffworks.com/adam-200132.htm

#### Phases of the Menstrual Cycle

As shown in Figure below, the menstrual cycle occurs in several phases. The cycle begins with menstruation. During menstruation, arteries that supply the endometrium of the uterus constrict. As a result, the endometrium breaks down and detaches from the uterus. It passes out of the body through the vagina over a period of several days.

Phases of the Menstrual Cycle. The menstrual cycle occurs in the phases shown here.

After menstruation, the endometrium begins to build up again. At the same time, a follicle starts maturing in an ovary. Ovulation occurs around day 14 of the cycle. After it occurs, the endometrium continues to build up in preparation for a fertilized egg. What happens next depends on whether the egg is fertilized.

If the egg is fertilized, the endometrium will be maintained and help nourish the egg. The ruptured follicle, now called the corpus luteum, will secrete the hormone progesterone. This hormone keeps the endometrium from breaking down. If the egg is not fertilized, the corpus luteum will break down and disappear. Without progesterone, the endometrium will also break down and be shed. A new menstrual cycle thus begins.

#### Menopause

For most women, menstrual cycles continue until their mid- or late- forties. Then women go through menopause, a period during which their menstrual cycles slow down and eventually stop, generally by their early fifties. After menopause, women can no longer reproduce naturally because their ovaries no longer produce eggs.

### Summary

• The menstrual cycle includes events that take place in the ovary, such as ovulation.
• The menstrual cycle also includes changes in the uterus, including menstruation.
• Menopause occurs when menstruation stops occurring, usually in middle adulthood.

### Practice

Use these resources to answer the questions that follow.

• http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Hormonal Control of Reproduction
1. What is estrogen?
2. What does GnRH do?
3. Describe the roles of FSH and LH in females.
4. Distinguish between a menstrual and estrus cycle.
5. Distinguish between the menstrual cycle and ovarian cycle.
6. Describe hormone levels during the follicular phase.
7. Describe hormone levels during the luteal phase.
8. Describe how birth control pills work.
1. What is the menstrual cycle?
2. What happens during days 1-5 of the menstrual cycle?
3. Describe what happens during the follicular phase of the menstrual cycle.
4. What are the roles of FSH and LH?
5. Describe what happens during the ovulatory phase of the menstrual cycle.
6. Describe what happens during the luteal phase of the menstrual cycle.
7. Describe the two possible fates of the corpus luteum.

### Review

1. Define menstruation. What is the first menstrual period called?

2. What is menopause? When does it occur?

3. Make a cycle diagram to represent the main events of the menstrual cycle in both the ovaries and the uterus, including the days when they occur.

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

### Vocabulary Language: English Spanish

corpus luteum

Remains of the follicle after ovulation.

endometrium

Tissues lining the uterus; shed from the body during menstruation.

follicle

Structure in the ovary where eggs mature.

menopause

Period during which menstrual cycles slow down and eventually stop in middle adulthood.

menstrual cycle

Monthly cycle of processes and events in the ovaries and uterus of a sexually mature human female.

menstruation

Process in which the endometrium of the uterus is shed from the body during the first several days of the menstrual cycle; also called monthly period.

ovulation

Release of a secondary oocyte from the uterus about half way through the menstrual cycle.

progesterone

A hormone that keeps the endometrium from breaking down.

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