Progesterone is a steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. Progesterone is the most potent natural progestational hormone. During pregnancy, progesterone is also produced by the placenta. Progesterone regulates the female reproductive functions together with estrogen and plays a particular role in the maintenance of pregnancy. Progesterone induces the cyclic changes in the endometrium that allow implantation of the fertilized ovum. Progesterone is also responsible for maintenance of the uterus during pregnancy, suppression of uterine contractions until just prior to parturition and preparation of the breasts for lactation.
Progesterone serves to thicken the uterine lining, preparing the uterus to support the fertilized egg, or zygote. Progesterone, along with estrogen, prepares the uterus (womb) for a possible pregnancy each month and supports the fertilized egg if conception occurs. Progesterone also helps prepare the breasts for milk production and breastfeeding. Progesterone is used to prevent overgrowth of the endometrium of postmenopausal women who have not had a hysterectomy and are receiving estrogen replacement therapy. Progesterone is also used to treat the absence of menstruation due to progesterone deficiency. Progesterone is a female hormone important for the regulation of ovulation and menstruation. In men, progesterone probably has no normal function except as an intermediate in the synthesis of other steroid hormones. In women, progesterone prepares the uterus for pregnancy and the breasts for milk production. After ovulation, progesterone blocks proliferation of the endometrium and stimulates the uterus to prepare for implantation of a fertilized egg. Progesterone levels continue to rise in early pregnancy.