Hormones of Human Pregnancy and Birth

Hormones of Human Pregnancy and Birth
If fertilization occurs, it normally does so in the first third of the uterine tube (ampulla), and the zygote travels from here to the uterus, dividing by mitosis to form a blastocyst (see Principles of Development) by the time it reaches the uterus. The developing blastocyst will contact the uterine surface after about 6 days and bury itself in the endometrium. This process is called implantation. Growth of the embryo continues, producing a spherically shaped trophoblast. This embryonic stage contains three distinct tissue layers, the amnion, chorion, and embryo proper, the inner cell mass (Figure 8-23). The chorion becomes the source of human chorionic gonadotropin (hCG), which appears in the bloodstream soon after implantation. hCG stimulates the corpus luteum to synthesize and release both estrogen and progesterone (Figure 7-16).

The point of attachment between trophoblast and uterus becomes the placenta (evolution and development of the placenta is described in See: Principles of Development). Besides serving as a medium for the transfer of materials between maternal and fetal bloodstreams, the placenta also serves as an endocrine gland. The placenta continues to secrete hCG and also produces estrogen (mainly estriol) and progesterone. After about the third month of pregnancy, the corpus luteum degenerates, but by then the placenta itself is the main source of both progesterone and estrogen (Figure 7-17).

Preparation of the mammary glands for secretion of milk requires two additional hormones, prolactin (PRL) and human placental lactogen (hPL) (or human chorionic somatomammotropin). PRL is produced by the anterior pituitary, but in nonpregnant women its secretion is inhibited. During pregnancy, elevated levels of progesterone and estrogen depress the inhibitory signal, and PRL begins to appear in the blood. PRL, in combination with hPL, prepare the mammary glands for secretion. hPL, together with maternal growth hormone, also stimulates an increase in available nutrients in the mother, so that more are provided to the developing embryo. Later the placenta begins to synthesize a peptide hormone called relaxin; this hormone allows some expansion of the pelvis by increasing the flexibility of the pubic symphysis, and also dilates the cervix in preparation for delivery.

Birth, or parturition, begins with a series of strong, rhythmic contractions of the uterine musculature, called labor. The exact signal that triggers birth is not fully understood in humans, but several important factors have been identified in other mammals. Just before birth, secretion of estrogen, which stimulates uterine contractions, rises sharply, while the level of progesterone, which inhibits uterine contractions, declines (Figure 7-17). This removes the “progesterone block” that keeps the uterus quiescent throughout pregnancy. Prostaglandins, a large group of hormones (long-chain fatty acid derivatives), also increase at this time, making the uterus more “irritable” (See: Chemical Coordination, for more on prostaglandins). Finally, stretching of the uterus sets in motion neural reflexes that stimulate secretion of oxytocin from the posterior pituitary. Oxytocin also stimulates uterine smooth muscle, leading to stronger and more frequent labor contractions.

Given the intricacy of pregnancy it may seem remarkable that healthy babies are ever born! In fact we are the lucky survivors of pregnancy, for miscarriages are quite common and serve as a mechanism to reject prenatal abnormalities such as chromosomal damage and other genetic errors, exposure to drugs or toxins, immune irregularities, or improper hormonal priming of the uterus. Modern hormonal tests show that about 30 percent of fertile zygotes are spontaneously aborted before or right after implantation; such miscarriages are unknown to the motheror are expressed as a slightly late menstrual period.Another 20 percent of established pregnancies end in miscarriage (those known to the mother), giving a spontaneous abortion rate of about 50 percent.

Childbirth occurs in three stages. In the first stage the neck (cervix), or opening of the uterus into the vagina, is enlarged by pressure from the baby in its bag of amniotic fluid, which may be ruptured at this time (Figure 7-18B). In the second stage, the baby is forced out of the uterus and through the vagina to the outside (Figure 7-18C). In the third stage, the placenta, or afterbirth, is expelled from the mother’s body, usually within 10 minutes after the baby is born (Figure 7-18D).

After birth, secretion of milk is triggered when the infant sucks on its mother’s nipple. This leads to a reflex release of oxytocin from the pituitary; when oxytocin reaches the mammary glands it causes contraction of smooth muscles lining ducts and sinuses of the mammary glands and ejection of milk. Suckling also stimulates release of prolactin from the anterior pituitary gland, which stimulates continued production of milk by the mammary glands.