Why Does the Menstrual Cycle Cease During Pregnancy?
One of the most common signs that you're pregnant is a missed menstrual period. Although you may be familiar with this sign of pregnancy, many women don't understand exactly why the menstrual cycle ceases during the months of pregnancy. The answer lies in the fact that once you conceive, the intricate relationship between hormones that takes place during your menstrual cycle changes drastically to allow your body to provide for the needs of your developing baby.
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The Menstrual Cycle
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In order to fully understand why the menstrual cycle ceases during pregnancy, you should have a basic understanding of your menstrual cycle. You can determine your menstrual cycle by counting from the first day of your last period to the first day of the following menstrual period. According to the U.S. Department of Health and Human Services, the average woman's cycle lasts approximately 28 days. The American Pregnancy Association states that most women ovulate some time between day 11 and day 21 of the menstrual cycle.
Follicular Stage of the Menstrual Cycle
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The purpose of the menstrual cycle is to prepare your body for pregnancy. During the first half of your menstrual cycle (the follicular stage), your brain's pituitary gland releases certain female hormones into your bloodstream that cause follicles (sacs in your ovaries that contain immature eggs) to mature. These maturing follicles release increasing amounts of estrogen into your bloodstream, which makes the lining of your uterus (endometrium) thicken and eventually causes the most mature follicle to burst open, releasing its egg in the event known as ovulation. This empty follicle triggers the production of progesterone, a female hormone that causes other changes in your body to facilitate fertilization and implantation.
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Fertilization
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According to the American Pregnancy Association, once you ovulate, the mature egg travels down your fallopian tube and is available to be fertilized for a period of approximately 12 to 24 hours. You are most likely to conceive if you have intercourse some time in the three days prior to ovulation or on the day of ovulation itself. In the event that your egg is fertilized by sperm, the fertilized egg continues its journey to the uterus, at which point it attaches to the lining of your uterus. If the egg is not fertilized, your body's levels of estrogen and progesterone drop, causing menstruation to occur and thus shedding the lining of the uterus.
Implantation
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Once the fertilized egg attaches to the thickened lining of your uterus, which typically occurs six to 12 days after ovulation, your body shifts gears; your menstrual cycle has completed its job and is no longer required. Implantation triggers your body to produce human chorionic gonadotropin (hCG), a hormone present only during pregnancy that allows your body to continue producing progesterone and estrogen. The presence of hCG in your urine gives a positive pregnancy result on home pregnancy tests.
Inhibited Ovulation
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High levels of estrogen and progesterone are necessary for your pregnancy to continue. ExpectantMothersGuide.com states that estrogen causes important changes such as an increase in the flow of blood to your uterus, while progesterone is responsible for other tasks, including inhibiting uterine contractions, which allow your pregnancy to develop to term. According to the National Women's Health Resource Center, the high levels of these two hormones that occur during pregnancy inhibit your body from ovulating until your pregnancy has completed.
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