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Chapter: Medical Physiology: Pregnancy and Lactation

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Increased Uterine Excitability Near Term, Onset of Labor-A Positive Feedback Mechanism for Its Initiation.


Increased Uterine Excitability Near Term

Parturition means birth of the baby. Toward the end ofpregnancy, the uterus becomes progressively more excitable, until finally it develops such strong rhythmi-cal contractions that the baby is expelled. The exact cause of the increased activity of the uterus is not known, but at least two major categories of effects lead up to the intense contractions responsible for parturi-tion: (1) progressive hormonal changes that cause increased excitability of the uterine musculature, and (2) progressive mechanical changes.

Hormonal Factors That Increase Uterine Contractility

Increased Ratio of Estrogens to Progesterone. Progesteroneinhibits uterine contractility during pregnancy, thereby helping to prevent expulsion of the fetus. Conversely, estrogens have a definite tendency to increase the degree of uterine contractility, partly because estrogens increase the number of gap junctions between the adjacent uterine smooth muscle cells, but also because of other poorly understood effects. Both progesterone and estrogen are secreted in progres-sively greater quantities throughout most of preg-nancy, but from the seventh month onward, estrogen secretion continues to increase while progesterone secretion remains constant or perhaps even decreases slightly. Therefore, it has been postulated that the estrogen-to-progesterone ratioincreases sufficientlytoward the end of pregnancy to be at least partly responsible for the increased contractility of the uterus.


Effect of Oxytocin on the Uterus. Oxytocin is a hormonesecreted by the neurohypophysis that specifically causes uterine contraction. There are four reasons to believe that oxytocin might be impor-tant in increasing the contractility of the uterus near term: (1) The uterine muscle increases its oxytocin receptors and, therefore, increases its responsiveness to a given dose of oxytocin during the latter few months of pregnancy. (2) The rate of oxytocin secre-tion by the neurohypophysis is considerably increased at the time of labor. (3) Although hypophysectomized animals can still deliver their young at term, labor is prolonged. (4) Experiments in animals indicate that irritation or stretching of the uterine cervix, as occurs during labor, can cause a neurogenic reflex through the paraventricular and supraoptic nuclei of the hypothalamus that causes the posterior pituitary gland (the neurohypophysis) to increase its secretion of oxytocin.

Effect of Fetal Hormones on the Uterus. The fetus’s pitu-itary gland secretes increasing quantities of oxytocin, which might play a role in exciting the uterus. Also, the fetus’s adrenal glands secrete large quantities of corti-sol, another possible uterine stimulant. In addition, the fetal membranes release prostaglandins in high con-centration at the time of labor. These, too, can increase the intensity of uterine contractions.

Mechanical Factors That Increase Uterine Contractility Stretch of the Uterine Musculature. Simply stretchingsmooth muscle organs usually increases their contrac-tility. Further, intermittent stretch, as occurs repeat-edly in the uterus because of fetal movements, can also elicit smooth muscle contraction. Note especially that twins are born, on average, 19 days earlier than a single child, which emphasizes the importance of mechanical stretch in eliciting uterine contractions.

Stretch or Irritation of the Cervix. There is reason tobelieve that stretching or irritating the uterine cervix is particularly important in eliciting uterine contrac-tions. For instance, the obstetrician frequently induces labor by rupturing the membranes so that the head of the baby stretches the cervix more forcefully than usual or irritates it in other ways.

The mechanism by which cervical irritation excites the body of the uterus is not known. It has been suggested that stretching or irritation of nerves in the cervix initiates reflexes to the body of the uterus, but the effect could also result simply from myogenic transmission of signals from the cervix to the body of the uterus.

Onset of Labor-A Positive Feedback Mechanism for Its Initiation

During most of the months of pregnancy, the uterus undergoes periodic episodes of weak and slow rhyth-mical contractions called Braxton Hicks contractions.

These contractions become progressively stronger toward the end of pregnancy; then they change sud-denly, within hours, to become exceptionally strong contractions that start stretching the cervix and later force the baby through the birth canal, thereby causing parturition. This process is calledlabor, and the strong contractions that result in final parturition are called labor contractions.

We do not know what suddenly changes the slow, weak rhythmicity of the uterus into strong labor con-tractions. However, based on experience with other types of physiological control systems, a theory has been proposed for explaining the onset of labor. The positive feedback theory suggests that stretching ofthe cervix by the fetus’s head finally becomes great enough to elicit a strong reflex increase in contractil-ity of the uterine body. This pushes the baby forward, which stretches the cervix more and initiates more positive feedback to the uterine body. Thus, the process repeats until the baby is expelled. This theory is shown in Figure 82–9, and the observations sup-porting it are the following.

First, labor contractions obey all the principles of positive feedback. That is, once the strength of uterine contraction becomes greater than a critical value, each contraction leads to subsequent contractions that become stronger and stronger until maximum effect is achieved.

Second, two known types of positive feedback increase uterine contractions during labor: (1) Stretch-ing of the cervix causes the entire body of the uterus to contract, and this contraction stretches the cervix even more because of the downward thrust of the baby’s head. (2) Cervical stretching also causes the pituitary gland to secrete oxytocin, which is another means for increasing uterine contractility.

To summarize, we can assume that multiple factors increase the contractility of the uterus toward the end of pregnancy. Eventually a uterine contraction becomes strong enough to irritate the uterus, espe-cially at the cervix, and this increases uterine contractility still more because of positive feedback, resulting in a second uterine contraction stronger than the first, a third stronger than the second, and so forth. 

Once these contractions become strong enough to cause this type of feedback, with each succeeding con-traction greater than the preceding one, the process proceeds to completion—all because positive feedbackinitiates a vicious circle when the gain of the feedback is greater than a critical level.

One might ask about the many instances of false labor, in which the contractions become stronger and stronger and then fade away. Remember that for a vicious circle to continue, each new cycle of the posi-tive feedback must be stronger than the previous one. If at any time after labor starts some contractions fail to re-excite the uterus sufficiently, the positive feed-back could go into a retrograde decline, and the labor contractions would fade away.

Abdominal Muscle Contractions During Labor

Once uterine contractions become strong during labor, pain signals originate both from the uterus itself and from the birth canal. These signals, in addition to causing suffering, elicit neurogenic reflexes in the spinal cord to the abdominal muscles, causing intense contractions of these muscles. The abdominal contrac-tions add greatly to the force that causes expulsion of the baby.

Mechanics of Parturition

The uterine contractions during labor begin mainly at the top of the uterine fundus and spread downward over the body of the uterus. Also, the intensity of contraction is great in the top and body of the uterus but weak in the lower segment of the uterus adjacent to the cervix. Therefore, each uterine contraction tends to force the baby downward toward the cervix.

In the early part of labor, the contractions might occur only once every 30 minutes. As labor progresses, the contractions finally appear as often as once every 1 to 3 minutes, and the intensity of contraction increases greatly, with only a short period of relaxation between contractions. The combined contractions of the uterine and abdominal musculature during delivery of the baby cause a downward force on the fetus of about 25 pounds during each strong contraction.

It is fortunate that the contractions of labor occur intermittently, because strong contractions impede or sometimes even stop blood flow through the placenta and would cause death of the fetus if the contractions were continuous. Indeed, overuse of various uterine stimulants, such as oxytocin, can cause uterine spasm rather than rhythmical contractions and can lead to death of the fetus.

In about 95 per cent of births, the head is the first part of the baby to be expelled, and in most of the remain-ing instances, the buttocks are presented first. The head acts as a wedge to open the structures of the birth canal as the fetus is forced downward.

The first major obstruction to expulsion of the fetus is the uterine cervix. Toward the end of pregnancy, the cervix becomes soft, which allows it to stretch when labor contractions begin in the uterus. The so-called firststage of labor is a period of progressive cervical dilation,lasting until the cervical opening is as large as the head of the fetus. This stage usually lasts for 8 to 24 hours in the first pregnancy but often only a few minutes after many pregnancies.

Once the cervix has dilated fully, the fetal membranes usually rupture and the amniotic fluid is lost suddenly through the vagina. Then the fetus’s head moves rapidly into the birth canal, and with additional force from above, it continues to wedge its way through the canal until delivery is effected. This is called the second stageof labor, and it may last from as little as 1 minute aftermany pregnancies to 30 minutes or more in the first pregnancy.

Separation and Delivery of the Placenta

For 10 to 45 minutes after birth of the baby, the uterus continues to contract to a smaller and smaller size, which causes a shearing effect between the walls of the uterus and the placenta, thus separating the placenta from its implantation site. Separation of the placenta opens the placental sinuses and causes bleeding. The amount of bleeding is limited to an average of 350 mil-liliters by the following mechanism: The smooth muscle fibers of the uterine musculature are arranged in figures of eight around the blood vessels as the vessels pass through the uterine wall. Therefore, contraction of the uterus after delivery of the baby constricts the vessels that had previously supplied blood to the pla-centa. In addition, it is believed that vasoconstrictor prostaglandins formed at the placental separation site cause additional blood vessel spasm.

Labor Pains

With each uterine contraction, the mother experiences considerable pain. The cramping pain in early labor is probably caused mainly by hypoxia of the uterine muscle resulting from compression of the blood vessels in the uterus. This pain is not felt when the visceral sensory hypogastric nerves,which carry the visceral sensory fibers leading from the uterus, have been sectioned.

However, during the second stage of labor, when the fetus is being expelled through the birth canal, much more severe pain is caused by cervical stretching, per-ineal stretching, and stretching or tearing of structures in the vaginal canal itself. This pain is conducted to the mother’s spinal cord and brain by somatic nerves instead of by the visceral sensory nerves.

Involution of the Uterus After Parturition

During the first 4 to 5 weeks after parturition, the uterus involutes. Its weight becomes less than half its immedi-ate postpartum weight within 1 week, and in 4 weeks, if the mother lactates, the uterus may become as small as it was before pregnancy. This effect of lactation results from the suppression of pituitary gonadotropin and ovarian hormone secretion during the first few months of lactation, as discussed later. During early involution of the uterus, the placental site on the endometrial surface autolyzes, causing a vaginal discharge known as “lochia,” which is first bloody and then serous in nature, continuing for a total of about 10 days. After this time, the endometrial surface becomes re-epithelialized and ready for normal, nongravid sex life again.

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