The Role of Environmental Hazards in Premature Birth Workshop Summary (2003) / Chapter Skim
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2. Labor and Delivery
2. Labor and Delivery
Pages 27-41
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From page 27... ...
The only certain ultimately preterm birth, researchers have spent many years looking for the trigger that starts parturition without success. In the normal pregnancy, there is no trigger for parturition, according to Peter Nathanielsz, Cornell University.
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From page 28... ...
Animal studies have also provided clues to the mechanisms of myometrial activity or contraction, which has both a fetal and a maternal component. In all mammalian species studied, myometrial activity throughout pregnancy is of the contracture type, exemplified by long-lasting, low-frequency epochs of activity that have a very different temporal and amplitude pattern from contractions (see Figure 2.2~.
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From page 29... ...
By understanding the switch in myometrial activity and the interactions with environmental factors, researchers may better understand mechanism involved in preterm birth. Despite these observations, available evidence suggests that home uterine monitoring is of no value in predicting risk for preterm birth, especially since there are no effective interventions currently.
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From page 30... ...
In a series of studies in the pregnant rhesus monkey, Nathanielsz observed that situations resulting in elevated maternal estrogen concentrations in late pregnancy were generally accompanied by a switch in myometrial activity from the contracture to the contraction mode. Such situations included laparotomy and fetal catheterization surgery, hypoglycemia induced by food withdrawal, and the administration of androstenedione to the pregnant monkey.
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From page 31... ...
As discussed in the previous section, CAP genes are involved in the onset of labor and their expression increases as myometrial activation occurs. Lye suggested that estrogen increases the expression of CAP genes, The rise in estrogen is the central key for normal labor and delivery, a context within which it becomes interesting to look at environmental factors and environmental disruptions, including stress hormones.
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From page 32... ...
These findings have led some to believe that the fetus determines the length of ovine pregnancy. The endocrine pathway involves activation of the fetal HPA axis leading to increased synthesis and release of cortisol from the fetal adrenal gland.
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From page 33... ...
In turn, this enzyme is unregulated by the fetal membranes by prostaglandins, forming a feedback loop leading to increased prostaglandin production. Cortisol also increases expression of placental corticotropin-releasing hormone, which can in turn increase prostaglandin production.
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From page 34... ...
The genes in the empty horn were not turned on even though the empty horn was exposed to the endocrine changes, reported Lye, but if a tube was inserted in the empty horn to stretch the muscle, then all the labor genes were turned on. Interestingly, if stretch activates genes that cause labor, the question remains why animals don't trigger labor as the fetus grows.
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From page 35... ...
The magnitude of contraction is dependent on the extent of propagation of these electrical signals throughout the uterus to recruit additional smooth muscle cells. In order to have these smooth, coordinate muscle contractions, the myometrial cells must have rhythmic coupling to provide the contractility necessary for delivery.
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From page 36... ...
Although calcium-calmodulin mediates the major pathway associated with uterine contractions, there are opportunities for other pathways to influence the activities of various enzymes. The uterine relaxant pathways involve activation of cGMP cyclic guanosine 5'-monophosphate)
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From page 37... ...
Several studies have reported increases in NO synthesis in the uterus and in NO-induced uterine relaxation during pregnancy. Both NO synthesis and uterine sensitivity to NO are substantially reduced at term, indicating a role for NO in uterine quiescence during pregnancy and labor (Figure 2.7~.
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From page 38... ...
Progesterone not only enhances NO synthase expression and NO synthesis in the rat uterus but also enhances NO-induced uterine relaxation. On the other hand, antiprogesterones decrease NO synthase enzymes, inhibit NO synthesis, and reduce uterine relaxation responsiveness to NO.
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From page 39... ...
CGRP binding to myometrium is increased with pregnancy and decreased with labor. Moreover, CGRP receptor levels in myometrium parallel CGRP binding and relaxation responses.
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From page 40... ...
Although there are a number of smooth muscle cells, fibroblasts, and mast cells in the cervix, it is primarily comprised of collagen-type connective tissue. At the time of parturition, the breakdown in the cervical matrix coincides with a decrease in the amount of collagen and a subsequent increase in the water content.
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From page 41... ...
When an inflammation or breakdown of the extracellular matrix occurs, fibronectin leaks down into the cervix and the vagina. Goldenberg stated that women in early preterm labor with a negative fetal fibronectin test have a less than 1 percent chance of delivering in the subsequent two weeks.
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