The body secretes the female hormones estrogen and progesterone as a way of regulating various tissue functions. These substances are produced primarily by the ovaries and the adrenal glands in women, and in the adrenals and the testes in men. Estrogen and progesterone work together on several fronts, including controlling the reproductive cycle and preparing a woman’s body for pregnancy.
Estrogen is secreted by the ovaries to promote ovulation, which is the release of a mature egg that may be fertilized. This time during the cycle is known as the follicular phase. The body secretes estrogen during the follicular phase to develop the lining of the uterus, in case pregnancy occurs after ovulation. The thickened uterine lining promotes implantation of a fertilized egg.
The phase following the follicular phase is known as the luteal phase, and it is in this phase that the body begins to secrete more progesterone. This hormone also works to increase blood vessel growth in the uterus, increasing and thickening the uterine lining. During the luteal phase, both estrogen and progesterone are secreted by the corpus luteum, the leftover shell of the follicle that remains after the egg is released.
If pregnancy occurs, progesterone is necessary to help form the placenta. Progesterone also works to prevent uterine contractions in pregnancy, which protects the developing baby. After approximately three months of pregnancy, the developing placenta takes over production of estrogen and progesterone from the corpus luteum.
During pregnancy, progesterone and estrogen prepare a woman’s breasts for lactation and the ability to nurse a baby after delivery. Progesterone increases the development of cells that produce breast milk, while estrogen promotes growth of milk ducts. Estrogen also increases the body’s production of prolactin, a hormone that supports the production of breast milk.
If pregnancy does not occur after the luteal phase, estrogen and progesterone levels decline. The blood flow progesterone had originally promoted through blood vessel growth tapers off. Menstruation, which happens when the body sheds the uterine lining that estrogen and progesterone worked to build, then occurs. Following menstruation, the follicular cycle begins again. The entire cycle, from Day 1 of menstruation through the day the luteal phase ends just before the start of the next menstruation, typically runs 28 days.
Some women take estrogen as a form of hormone replacement therapy to reduce unpleasant symptoms associated with menopause. During menopause, the ovaries secrete less estrogen, causing symptoms such as dry mouth, vaginal dryness, hot flashes and mood swings. Estrogen replacement may be given to reduce the incidence of symptoms and to help protect against osteoporosis. Progesterone replacement may be added to estrogen therapy for women who have not had a hysterectomy to help control the overgrowth of uterine tissue lining and the potential for uterine cancer.