43.7B: Labor and Birth
- Describe the process of labor and birth in humans
Labor and Birth
Labor is the physical effort of expulsion of the fetus and the placenta from the uterus during birth (parturition). The total gestation period from fertilization to birth is about 38 weeks (birth usually occurring 40 weeks after the last menstrual period). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch, sending nerve impulses to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm, a process that may take many hours, especially in a woman bearing her first child. At some point, the amniotic sac bursts and the amniotic fluid escapes. During stage two, the baby is expelled from the uterus with the umbilical cord still attached. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall, usually within a few minutes. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary, which signals the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.
Key Points
- At the end of gestation, estrogen receptors on the uterine wall bind oxytocin, which causes the uterine muscles to contract; as the muscles contract, they signal for the release of more oxytocin in a positive feedback loop.
- During the first stage of labor, the cervix thins and dilates to allow passage of the baby into the birth canal; typically over the course of several hours, the cervix will dilate to its full width of 10 centimeters.
- During the second stage of labor, contractions become very strong and, aided by the pushing of the mother, the baby is expelled from the uterus.
- During the third stage of labor, the placenta, amniotic sac, and the remainder of the umbilical cord are expelled from the uterus, usually within a few minutes after birth.
- When the baby begins suckling at the breast after delivery, prolactin signals the release of milk from the mammary glands, providing nutrition and immunity against disease to the infant.
Key Terms
- prolactin : a peptide gonadotrophic hormone secreted by the pituitary gland; it stimulates growth of the mammary glands and lactation in females
- parturition : the act of giving birth; childbirth
- oxytocin : a hormone that stimulates contractions during labor, and then the production of milk