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Progesterone is especially important during pregnancy.
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In the part of her life between puberty and menopause, a woman experiences a monthly cycle regulated by a complex system of different hormones. One of these hormones, progesterone is especially important after ovulation in preparing the uterus for an embryo if conception occurs and in maintaining the uterus during an ensuing pregnancy.
Chemically, progesterone is a fat-soluble molecule called a steroid. During the first half of a woman's menstrual cycle, which typically takes 28 days, a structure called a follicle in the ovary produces large amounts of another female hormone, estrogen, and a small amount of progesterone.
At the middle of the cycle, an increase in a pituitary hormone called LH causes the follicle to release an egg, known as ovulation, and also causes the remaining parts of the follicle to change into a structure called a corpus luteum. Cells in the corpus luteum begin to make more and more progesterone and release it into the blood, where it reaches a maximum level around the 21st day of the cycle.
If conception doesn't occur, the progesterone level starts to fall and drops back to low levels by about day 26 of the cycle. Without conception, the corpus luteum dies, menstrual flow begins and the ovary prepares to begin the cycle again the following month.
Progesterone exerts positive effects on several structures during the second half of the menstrual cycle. After ovulation, the egg passes into the fallopian tube, where it stays for about 80 hours and is fertilized if sperm are present. At the end of that time, rising levels of progesterone cause muscle in the tube's wall, at its junction with the uterus, to relax, allowing the egg to move into the uterus.
During the next part of the menstrual cycle, progesterone has a major effect on the uterus, which prepares to support an embryo after it implants. Progesterone inhibits contraction of the smooth muscle in the wall of the uterus, helping keep the organ relaxed in the event of a pregnancy. It also causes cells lining glands in the wall of the uterus to produce a fluid that contains carbohydrate, fat and protein, which an embryo needs for nutrition during the first trimester. If conception doesn't take place during the cycle, the corpus luteum in the ovary dies, the progesterone level falls and the uterine lining partially regresses and is shed to become part of the menstrual flow.
Pregnancy and Progesterone
If conception occurs and the embryo implants successfully in the uterine wall, the placenta develops and makes a hormone called human chorionic gonadotropin. HCG stimulates the corpus luteum in the ovary, stopping it from degenerating and helping it continue making large amounts of progesterone.
Around the 10th week of pregnancy, the placenta begins making progesterone, helping keeping the mother's blood levels up to support the uterus and growing fetus. In addition to continuing its stimulation of uterine glands, during pregnancy progesterone suppresses development of other eggs in the ovary and helps prevent early uterine contractions, while thickening uterine muscle in preparation for birth. It also stimulates uterine cells to produce compounds called growth factors that are important for fetal development and it causes uterine blood vessels to grow as the uterus enlarges. Finally, progesterone causes glands in the cervix to make an especially thick mucus that forms a plug over the uterine opening, helping to exclude bacteria.
Birth and Possible Problems
At the time of birth, when the placenta begins loosening its attachment to the uterus, the progesterone level in the mother's blood falls. This helps trigger some of the events of birth, including uterine contractions and thinning of the cervical mucus.
Sometimes, problems that arise during pregnancy might be caused by insufficient production of progesterone, either by the corpus luteum during early pregnancy, or by the placenta later in pregnancy. Low levels of progesterone can compromise the ability of the uterus to support the fetus and might contribute to a miscarriage. If placental progesterone is low later in pregnancy, this might contribute to pre-term, or premature, birth. In some cases, a doctor might recommend taking regular progesterone or a synthetic form of the hormone to supplement the body's own production.