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Progesterone’s Role in Pregnancy
The hormone progesterone is often called “the pregnancy hormone” because it plays 2 important roles in pregnancy:
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Progesterone thickens the endometrium
Throughout an average 28-day menstrual cycle, the thickness of the endometrium varies. (The menstrual cycle begins on the first day of a woman’s period. For more information on a normal menstrual cycle, click here.)
During the first 14 days of the menstrual cycle, the endometrium is relatively thin and begins to thicken before ovulation. After ovulation, the endometrium continues to thicken. If no pregnancy occurs in this cycle, the endometrium will shed during a woman’s period and become thinner. If pregnancy occurs, the endometrium will remain thick to support the pregnancy.
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The difference in the thickness of the endometrium varies depending upon the amount of progesterone in the body. The endometrium is thinnest during the first half of the menstrual cycle, when progesterone levels are low. The endometrium becomes thickest after ovulation when progesterone levels rise.
The increasing levels of progesterone after ovulation are important to prepare the uterus for implantation of a fertilized egg.
Progesterone supports pregnancy
After a fertilized egg implants in the endometrium, the ovaries continue to produce progesterone to provide a nourishing environment for the growing embryo.
During a normal pregnancy, around 8 weeks after implantation, the placenta takes over the production of progesterone from the ovaries. The placenta produces a significant level of progesterone to maintain a healthy pregnancy.
A developing human in the womb. The term embryo is used from implantation through 8 weeks gestation. After 8 weeks, the developing human is considered a fetus.
The inner lining of the uterus.
The process of attachment of a fertilized egg to the endometrium.
The monthly cycle that generally lasts for 28 days. The cycle begins on the first day of a woman’s period, and ovulation generally occurs around day 14. Not all cycles are 28 days. Regular menstrual cycles can range from 23 to 35 days.
A pair of female reproductive organs that develop and release eggs each month during the menstrual cycle. Some hormones, like estrogen and progesterone, are released from the ovaries at certain times during the menstrual cycle and pregnancy.
The release of a mature egg from the ovary.
The organ that develops during pregnancy that transports nutrients to the fetus and waste away from the fetus. The placenta is attached to the uterus and is connected to the fetus by the umbilical cord.
The muscular organ in which a fertilized egg implants and matures through pregnancy. During menstruation, the uterus sheds the inner lining.
A hormone that plays 2 important roles in pregnancy: 1) thickens the endometrium to prepare for implantation of a fertilized egg, and 2) supports pregnancy through birth.