Progesterone plays an important role during pregnancy. After delivery of the baby, the levels of this hormone begin to drop. Breastfeeding causes low levels of progesterone due to the effect of prolactin, which prevents ovulation. Thus, progesterone levels tend to be very low during this time. When a mother stops lactating, prolactin levels fall, ovulation resumes and progesterone forms again.
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Actions of Progesterone
Progesterone normally forms in the second half of the menstrual cycle, after ovulation. The most important function of progesterone is to cause changes in a woman's uterus and genital tract to allow for fertilization and implantation of her egg. At the end of each cycle, progesterone levels fall; when this happens, the excess tissue in the uterus is shed, leading to menstruation. Progesterone causes other effects as well, including growth of the breast, fluid retention and a slight rise in body temperature.
Removal of Placenta
Estrogen combines with progesterone to facilitate development of the uterus and growth of the breast. Progesterone is thought to play a role in preventing the uterus from contracting, thereby preventing premature delivery. Immediately after delivery of a baby, the levels of these maternal hormones fall drastically. Much of the progesterone formed during pregnancy is formed by the placenta. Removal of the placenta removes the source of progesterone, and levels begin to fall.
Breastfeeding and Progesterone
During pregnancy, the breasts grow in size due to the action of estrogen and progesterone. Milk secretion, however, begins only after delivery of the baby. Lactation is controlled by two hormones, prolactin and oxytocin. Milk forms because of prolactin, which is made in the mother's pituitary gland. The release of milk by an infant's suckling is due to another hormone, oxytocin. Besides its role in milk formation, prolactin also acts on the pituitary. There, it blocks the release of follicle-stimulating hormone, or FSH, and leutenizing hormone, or LH. These two hormones are responsible for stimulating the ovary to form follicles that ovulate. Neither estrogen nor progesterone forms during this time. Because ovulation is prevented during this time, breastfeeding can be a moderately effective form of contraception.
Because of the action of the breastfeeding hormone prolactin, the levels of progesterone are low during lactation. Doctors rarely measure these levels of progesterone; however, during the first few weeks of breastfeeding, they are less than 5 nanograms per milliliter. After about six to 10 weeks of breastfeeding, they may range from 5 to 10 ng per ml. In contrast, progesterone levels usually rise to more than 15 ng per ml during the second half of an ovulating woman's cycle.
Resumption of Ovulation
When a mother stops breastfeeding, her levels of prolactin begin to fall. This leads to a rise in FSH and LH, allowing the resumption of menstrual cycles. FSH and LH act on the ovaries to stimulate production of follicles. Once a follicle is fully formed, the egg is released through ovulation. The part of the follicle left behind forms the corpus luteum, which functions to form progesterone.