Why Is The Menstrual Cycle Suppressed During Pregnancy?
The first thing most women notice when they are pregnant is a missed menstrual cycle. It is a balance of hormones from the thyroid that controls the natural cycle each month when there is no pregnancy. These hormones are naturally increased during pregnancy and include estrogen, progesterone, human chorionic gonadotropic (hCG) and prolactin (or human placental lactogen--HPL). The increased levels of these hormones suppresses the menstrual cycle to accommodate the pregnancy.
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Before Pregnancy
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Before pregnancy, follicle stimulating hormone (FSH) rises until it causes estrogen to begin developing the ovum. Estrogen levels rise until it is ready to be released, then the pituitary gland releases luteinizing hormones (LH) to trigger the release of the egg and estrogen levels decrease.
Once the egg is fertilized, the hormones begin working to maintain a pregnancy, which also means suppressing the menstrual cycle in order to maintain the endometrial lining. The endometrial lining becomes important during pregnancy because the fertilized egg burrows into it. The lining provides nourishment to the growing egg and eventually turns into the umbilical connection between mother and child. The further suppression of the menstrual cycle is done by a combination of different reproductive hormones.
Estrogen
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Estrogen levels are relatively low at the time pregnancy occurs, but increase to allow extra blood flow to the uterus during pregnancy. The extra blood flow to the uterus provides nourishment to the growing fetus.
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Progesterone
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Progesterone is naturally produced by the ovaries and thickens the endometrial lining of the uterus once the egg implants. Progesterone levels are continuously monitored during pregnancy by doctors to ensure the pregnancy does not miscarry as elevated levels of progesterone prevent menstrual bleeding. Progesterone prevents prostaglandin production and allows the developing baby to grow. When a woman is not pregnant, prostaglandin causes the uterine cramping before and during the menstrual cycle. The increased progesterone levels also cause tenderness in the breasts that is commonly associated with pregnancy.
Progesterone levels will drop at the end of a pregnancy to trigger labor and the birth of the baby.
Human Chorionic Gonadotropic (HCG)
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Human chorionic gonadotropic (HCG) hormone is produced when a fertilized egg is implanted in the uterus. HCG continues to rise during pregnancy and is believed to be responsible for maintaining the correct progesterone levels to maintain pregnancy. HCG is the hormone detected by pregnancy tests and is thought to be responsible for early pregnancy symptoms like nausea, vomiting and fatigue.
Prolactin
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Prolactin, also known as human placental lactogen (HPL), is triggered by increasing estrogen levels. It induces lactation after delivery of the baby. It also plays a role in the proper development of the fetus.
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