Birth And Fetal Development

 Birth

The birth process, which is complex, is regulated by various hormones. When diminished levels of progesterone remove the inhibition on uterine contractions, the uterine muscles contract. The posterior pituitary gland releases the hormone oxytocin, which stimulates further contractions.

During the first stage of labor, the cervix opens and the baby descends into the birth canal, or vagina. By this time, the amniotic sac has broken. In the second stage of labor, the baby passes through the birth canal, assisted by painful uterine contractions. The baby’s head normally appears first, but in a breech birth the buttocks may appear first. In the third stage, the placenta (afterbirth) is delivered.

Fetal Development

During the third month, the fetus definitely resembles a human, but the head is relatively large. During the ensuing months, the remainder of the body increases in size proportionally. Cartilage is replaced by bone, and the reproductive organs develop.

During the fourth month, the length of the fetus increases to about 6 inches. The heartbeat can be heard through the mother’s abdominal wall, and the fetus moves about. Distinctive movements can be felt at the fifth month, and by the sixth month the fetus weighs almost 2 pounds. By the end of six months, the fetus might be able to survive outside the mother’s body, but it would have little fat in its skin, so temperature control would be a problem. By the end of the ninth month, the fetus has an average length of about 20 inches and a typical weight of 6 to 8 pounds.