Module 4 Lesson 4 - 2

The Birthing Process

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Read pages 523 - 526

Gestation takes approximately 266 days, at which time the pregnancy ends with the birth of the fetus. The human chorionic gonadotropin hormone (hCG) levels increase drastically in the first trimester, as shown in the graph (Graph 1). Initially, it is secreted by the embryo’s trophoblast, which is the outer layer of the blastocyst. It acts similar to luteinizing hormone (LH) and maintains the corpus luteum.

Keep in mind that progesterone maintains the endometrium to ensure that the pregnancy continues until the fetus reaches full-term. Without progesterone, the endometrium will begin to shed, initiating menstruation. Estrogen builds the endometrium and, together with progesterone, suppresses the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) to ensure that no other eggs mature and become fertilized during a pregnancy.

During the first trimester, progesterone and estrogen production are maintained by the corpus luteum. After the placenta matures at the beginning of the second trimester, it takes over the role of corpus luteum and assumes maintenance of the endometrium. The placenta continues to produce hCG at lower levels but secretes large quantities of progesterone and estrogen during the last six months of the pregnancy, as shown in the graph (Graph 1).

Lastly, relaxin is another hormone that is produced by corpus luteum and later by the placenta during pregnancy. Relaxin, as the name suggests, relaxes the ligaments and joints so that the body can accommodate the growing fetus and facilitate childbirth.

In addition to maintaining the endometrium, progesterone also prevents uterine contraction. The slight reduction in progesterone levels near the end of pregnancy signals labour to begin.

A few days before a baby is ready to enter the outside world, the mother feels the sensation of the baby settling deeper into her pelvis. This sensation is commonly called lightening. Parturition or labour (terms given to the birthing process) has 3 stages: the dilation stage, the expulsion stage, and the placental stage. A woman is said to be 'in labour' when the cervix is dilated to about 3 cm in diameter and the uterine contractions last for 40 seconds and occur every 15 to 20 minutes.

The figure 15.14 on page 523 of the textbook outlines positive feedback mechanisms that are believed to control parturition (Diagram 1).

Dilation Stage

Labour is controlled by both nerves and hormones through a positive feedback system. The dilation stage of parturition begins as the cervix is shortened, contracting up into the uterus (Image 1).  This causes neural pathways to stimulate the hypothalamus to trigger the release of the hormone oxytocin from the lobes of the posterior pituitary. The release of oxytocin causes uterine contractions to continue to push the fetus down toward the cervix. The cervix continues to stretch, which positively feeds back to the posterior pituitary to continue the release of oxytocin.  

The uterine contractions stimulate the release of prostaglandins by positive feedback. Prostaglandins stimulate the contraction of the smooth muscle cells of the uterus and induce labour.

Expulsion Stage

The expulsion stage of parturition consists of the delivery of the fetus (Image 2). This is the stage in which the baby is actually 'born'. For this to happen, the baby's head has to rotate as it moves into the birth canal. When the head and shoulders of the baby are out of the birth canal, the body delivers quickly. Soon after delivery, the baby’s umbilical cord can be cut.

A normal vaginal birth may not be possible for several reasons. When this occurs, doctors have to resort to a Caesarean section (Image 5). A C-section, as it is sometimes called, consists of a surgical procedure in which an incision is made in the mother's lower abdomen through to her uterus and the baby is removed.

Placental Stage

In the placental stage, which in most cases happens within 30 minutes after the baby is delivered, the placenta disengages from the uterus and is expelled from the body. Once the placenta is expelled, the levels of estrogen, progesterone, and human chorionic gonadotropin (hCG) hormone drop drastically, as shown in the graph above (Graph 1).

 

 

 

Lactation

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Lactation, or breastfeeding, occurs when estrogen and progesterone production is suppressed and the anterior pituitary secretes the hormone prolactin (Image 6). The suckling of the new baby stimulates breast milk production through nerve endings in the nipple. Nervous stimulation causes oxytocin to be secreted, which acts to cause contractions in the smooth muscles of breasts and the uterus. These uterine contractions help reduce blood loss and shrink the size of the uterus. Therefore, new mothers are encouraged to breastfeed as soon after delivery as possible.

For the first few days of breastfeeding, new mothers produce colostrum, a yellowish liquid that contains antibodies and more protein than normal breast milk does.

Lactation Feedback

Lactation occurs through a positive feedback (Diagram 2). Prolactin hormone signals milk production and milk is stored in the lobules.

The stored milk is released through this process:

Suckling → Nerve impulses → Hypothalamus → Oxytocin secretion (stored in posterior pituitary) → Posterior pituitary releases stored oxytocin → Blood → Breast alveoli smooth muscles → Muscle contraction → Milk letdown

Watch and Listen

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While viewing the following video clip, give particular attention to the roles of nerve stimulation and hormone control on both parturition and lactation. Take notes while viewing this video.