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Explanation for premature and delayed labor.

Untimely spurts of the hormone oxytocin may be to blame for premature labor, according to a study of pregnant rats. Similarly, delayed release of the hormone may postpone labor beyond the healthiest time to deliver, the study suggests.

These findings could lead to the development of drugs to prevent premature labor by blocking the action of oxytocin in the uterus. Prematurity is a major cause of death among newborns, because early uterine contractions often expel a fetus before its lungs and other vital organs have matured. Similarly, a delay in labor can cause suffocation as the fetus outgrows the placenta's oxygen-bearing capabilities. Delayed labor can also endanger the mother, causing complications such as vaginal ruptures because of the child's larger size.

For years, obstetricians have administered oxytocin to stimulate labor in women whose pregnancies extended beyond the normal 40 weeks. In most cases, shots of the hormone quickly bring on intense contractions, followed by birth. But physicians have remained stumped over oxytocin's normal function during childbirth, because women who enter labor spontaneously do not have significantly higher blood concentrations of the hormone than pregnant women who aren't in labor. Moreover, animal tests have shown that injections of antibodies that block oxytocin in the blood do not delay the onset of labor.

The new study - led by neuroendocrinologist Hans H. Zingg of the Royal Victoria Hospital at McGill University in Montreal -- resolves this paradox. In the June 12 SCIENCE, Zingg and his colleagues report that a gene in the uterus of pregnant rats that codes for the production of oxytocin becomes steadily more active during pregnancy, peaking just before labor at more than 150 times its normal level of expression. In contrast, the gene's activity in the hypothalamus -- the brain region previously believed responsible for making oxytocin -- rose less than threefold, they found.

Zingg's group concludes that the hypothalamus is relatively unimportant in regulating the oxytocin levels that prompt labor. Instead, they assert, unknown factors spur the uterus itself to make oxytocin, which then causes the organ to contract. Errors in this process may cause premature or delayed labor, they suggest.

Zingg says the production of oxytocin by the uterus explains the lack of elevated oxytocin concentrations in the blood of pregnant women. Similarly, he says, oxytocin-blocking antibodies would not delay labor, because such comparatively large antibodies cannot leave the blood and enter the uterus.

Bryan F. Mitchell, an obstetrician at the University of Alberta in Edmonton, says he and his colleagues have found evidence to support this conclusion in human placental tissue. They discovered that women who enter labor spontaneously have much higher placental concentrations of oxytocin messenger RNA -- which directs the production of the hormone -- than women whose babies are delivered by cesarean section before labor begins.
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Title Annotation:oxytocin hormone reactions
Author:Ezzell, Carol
Publication:Science News
Date:Jun 13, 1992
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