Endocrine System

The ovaries are located at the end of each fallopian tube in the female reproductive tract, and they produce the female reproductive hormones: estrogen, progesterone, and relaxin. Although the fluctuation of these hormones is critical to the female menstrual cycle, they are initially triggered by a hormone from the hypothalamus, called a releasing factor, that enables gonadotrophs in the pituitary to release LH and FSH that, in turn, regulate part of the menstrual cycle. All of these hormones work together as part of the endocrine system to ensure fertility. They are also important for the development of sexual characteristics during puberty.

Each month after puberty, females release a single egg (ovulation) when the pituitary releases LH. Prior to ovulation, the maturing egg releases increasing levels of estrogen that inform the pituitary to secrete LH. While an egg travels down the fallopian tube, progesterone is released which prevents another egg from beginning to mature. Once an egg is shed in the uterine lining (in menstruation), the cycle can begin again. During pregnancy, high levels of circulating estrogen and progesterone prevent another egg from maturing. Estrogen levels fall dramatically at menopause, signifying the end of menstrual cycling and fertility. Menopause usually occurs between the ages of 40 and 50.

Endocrine regulation of the female reproductive tract does not stop during pregnancy. In fact, more sex hormones are released during pregnancy than during any other phase of female life. At this time, a new set of conditions support and protect the developing baby. Even cells of the developing embryo begin to release some hormones that keep the uterine lining intact for the first couple of months of pregnancy. High progesterone levels prevent the uterus from contracting so that the embryo is not disturbed. Progesterone also helps to prepare breasts for lactation. Towards the end of pregnancy, high estrogen levels stimulate the pituitary to release the hormone, oxytocin, which triggers uterine contractions. Prior to delivery, the ovaries release the hormone, relaxin, a protein which causes the pelvic ligaments to become loose for labor.