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Women's Health

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Normal Menstrual Cycle - Topic Overview

What is a menstrual cycle?

The menstrual cycle camera.gif is the series of changes a woman's body goes through to prepare for a pregnancy. About once a month, the uterus grows a new lining (endometrium) to get ready for a fertilized egg camera.gif. When there is no fertilized egg to start a pregnancy, the uterus sheds its lining. This is the monthly menstrual bleeding camera.gif (also called menstrual period) that women have from their early teen years until menopause, around age 50.

The menstrual cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding. Although the average cycle is 28 days, it is normal to have a cycle that is shorter or longer.

Girls usually start having menstrual periods between the ages of 11 and 14. Women usually start to have fewer periods between ages 39 and 51. Women in their 40s and teens may have cycles that are longer or change a lot. If you are a teen, your cycles should even out with time. If you are nearing menopause, your cycles will probably get longer and then will stop.

Talk to your doctor if you notice any big change in your cycle. It's especially important to check with your doctor if you have three or more menstrual periods that last longer than 7 days or are very heavy. Also call if you have bleeding between your periods or pelvic pain that is not from your period.

What controls the menstrual cycle?

Your hormones control your menstrual cycle. During each cycle, your brain's hypothalamus and pituitary gland send hormone signals back and forth with your ovaries. These signals get the ovaries and uterus ready for a pregnancy.

The hormones estrogen and progesterone play the biggest roles in how the uterus changes during each cycle.

  • Estrogen builds up the lining of the uterus.
  • Progesterone increases after an ovary releases an egg (ovulation) at the middle of the cycle. This helps the estrogen keep the lining thick and ready for a fertilized egg.
  • A drop in progesterone (along with estrogen) causes the lining to break down. This is when your period starts.
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