Late in gestation in some species, there is a rise in maternal serum estrogen and a drop in progesterone levels. However, the ratio of these hormones remains unchanged in human serum. Nevertheless, there is a proposed "functional" progesterone withdrawal that involves changes in uterine progesterone receptor and progesterone metabolism.

Oxytocin, which stimulates powerful uterine contractions, is secreted from the pars nervosa (posterior pituitary). It plays a significant role in the progression and completion of parturition. The release of oxytocin occurs in response to cervix stretch through a neuroendocrine reflex. This hormone facilitates delivery by stimulating uterine contractions. While oxytocin can be used to induce parturition, it is not believed to initiate it since maternal serum oxytocin levels increase after parturition has begun. However, progesterone inhibits oxytocin synthesis while estrogen stimulates it by influencing the synthesis of oxytocin receptors.

During parturition, prostaglandins and other cytokines increase uterine motility, thereby aiding in delivery. The exact role of these compounds in initiating parturition is still unknown. Prior to the onset of labor, prostaglandin levels in amniotic fluid, fetal membranes, and uterine decidua increase. Prostaglandin F2α and prostaglandin E2 promote uterine motility and have been used in high doses to induce labor. Estrogens, which are present in higher levels late in gestation, stimulate the synthesis of prostaglandins in the uterus, amnion, and chorion.

The size of the uterus is considered a factor that regulates parturition because stretch of the smooth muscle, including the uterus, enhances muscle contraction. Uterine stretch also stimulates the production of uterine prostaglandins. Multiple births tend to occur prematurely, which could be due to increased uterine size, increased fetal production of chemicals that stimulate delivery, or both.
Although a rise in maternal serum estrogen and a drop in progesterone levels occur late in gestation in some species, no change in the ratio of the two hormones is seen in human serum. However, “functional” progesterone withdrawal involving changes in uterine progesterone receptor and progesterone metabolism has been proposed.
Oxytocin is secreted from the pars nervosa (posterior pituitary) (see ). Oxytocin, which stimulates powerful uterine contractions, plays a major role in progression and completion of parturition. Oxytocin is released in response to stretch of the cervix through a neuroendocrine reflex; it stimulates uterine contractions and thereby facilitates delivery. Oxytocin can be used to induce parturition, and uterine sensitivity to oxytocin increases before parturition. Because maternal serum oxytocin levels do not increase until after parturition has begun, oxytocin is not thought to initiate parturition. However, progesterone inhibits and estrogen stimulates synthesis of oxytocin receptors, and although maternal serum progesterone levels do not decrease immediately before human parturition, estrogen levels rise and oxytocin receptor synthesis increases.
Prostaglandins and other cytokines increase uterine motility, and levels of these compounds increase during parturition, thereby facilitating delivery. Their exact role in initiating parturition is not known. Prostaglandin levels in amniotic fluid, fetal membranes, and uterine decidua increase before the onset of labor. Prostaglandin F2α and prostaglandin E2 increase uterine motility. Large doses of these compounds have been used to induce labor. Because estrogens stimulate prostaglandin synthesis in the uterus, amnion, and chorion, the rising estrogen levels late in gestation can increase uterine prostaglandin formation before parturition.
Uterine size is thought to be a factor regulating parturition because stretch of smooth muscle, including the uterus, increases muscle contraction. In addition, uterine stretch stimulates uterine prostaglandin production. Multiple births generally occur prematurely. The tendency for early delivery can be a result of increased uterine size, increased fetal production of chemicals stimulating delivery, or both.
CHAPTER 44 The Male and Female Reproductive Systems 825 • Fig. 44.37 Diagram of the structure of the breast, along with some pathological conditions of the breast and where they occur. (From Crum CP et al. In: Kumar V et al [eds]. Robbins Basic Pathology. 7th ed. Philadelphia: Saunders; 2003.)
Structure of the Mammary Gland
The mammary gland is composed of 15 to 20 lobes, each with an excretory lactiferous duct that opens at the nipple (