The goal of this page is to gather together the information discussed in lecture about the functions of estrogen and progesterone at each step in pregnancy.
The normal pattern of gonadal steroid secretion is necessary to create the ideal circumstances for implantation. Increasing levels of estrogen during the follicular phase promote endometrial proliferation and induce expression of progesterone receptors. In the luteal phase, progesterone stimulates secretion by endometrial glands, and also promotes blood vessel growth.
In the luteal phase following ovulation, estrogen and progesterone are secreted by the corpus luteum, which forms through the action of luteinizing hormone (LH). The corpus luteum continues to be the source for estrogen and progesterone in early pregnancy. Growth of the corpus luteum is stimulated by chorionic gonadotropin (CG), a peptide hormone that is synthesized by the trophoblast as soon as the embryo implants. CG binds to the LH receptor, maintaining the corpus luteum in an active state throughout the first three months of pregnancy. Starting at about the third month of pregnancy, the placenta takes over as the source of estrogen and progesterone.
Throughout pregnancy, progesterone is important because it
prevents contractions by promoting smooth muscle relaxation in the myometrium.
Another important effect of progesterone is that it causes the
developmental changes in the endometrium that are necessary for
the formation of the decidua, the maternal portion of the
placenta. The progesterone antagonist mifepristone can be
used to terminate a pregnancy because it blocks progesterone
action and causes the uterus to contract. Supplemental
progesterone is a treatment that is used to prevent preterm birth
in women who have a history of preterm labor.
Both gonadal steroids help prepare a woman for motherhood by stimulating breast growth. Progesterone stimulates growth of milk-producing cells in the glands, while estrogen stimulates growth of duct cells. Estrogen further stimulates growth by stimulating secretion of prolactin, a hormone that promotes breast growth and milk production. However, high levels of estrogen and progesterone prevent lactation during pregnancy by inhibiting milk synthesis.
Parturition is the delivery of the infant followed by delivery of the placenta. Estrogen is the most important hormone in preparation for parturition. Estrogen stimulates proliferation in the myometrium, accomplishing the considerable growth that is necessary for the forceful contractions of labor. Importantly, as the time for parturition nears, estrogen stimulates the synthesis of gap junctions between myometrial smooth muscle cells. Gap junctions allow electrical activity to travel between cells, so that coordinated contractions can occur.
Near term, estrogen stimulates the synthesis of enzymes involved in prostaglandin synthesis. Before labor, prostaglandins stimulate cervical ripening, the breakdown of cervical connective tissue allowing it to become soft and flexible and capable of dilation. During labor, prostaglandins stimulate myometrial contractions.
The posterior pituitary hormone oxytocin is the strongest stimulator of uterine contractions. As the time of delivery approaches, estrogen increases responsiveness to oxytocin by increasing expression of oxytocin receptors.