NEW YORK - Pregnant women may go into labor because the fetus tells them to by sending a command from deep within the fetal brain, an animal study suggests.
The brain signal appears to set off a hormonal chain reaction that triggers labor, researchers said.
Further research might lead to earlier diagnosis of premature labor and better drugs to block some causes of premature birth, said study co-author Dr. Peter Nathanielsz.
Premature births occur in up to 10 percent of pregnancies and account for perhaps 75 percent of newborn deaths. Premature babies also run a higher risk of long-term trouble such as cerebral palsy and chronic lung problems.
The new study was done in sheep. Nathanielsz and Dr. Thomas McDonald, both of Cornell University in Ithaca, N.Y. report their findings in the September issue of the American Journal of Obstetrics and Gynecology.
The signal to the mother appears to come from a pair of brain centers called the paraventricular nuclei, they said. Those nuclei or other parts of the fetal brain may keep tabs on the fetus's development so that childbirth is triggered when the fetus is ready, researchers said.
"You would expect the fetus to be monitoring the development of those systems it needs to survive, then when a certain level of maturation is reached, the system is told, `Go,' " McDonald said.
Scientists do not know what initially triggers labor in people. The new study "lends more credibility to the concept that the fetus determines when labor starts," said Dr. Roger Freeman, professor of obstetrics and gynecology at the University of California at Irvine.
"There's at least as much evidence that the fetus initiates labor as the mother," Freeman said. For example, in a condition in which a fetus lacks much of its brain, the mother frequently does not go into labor, he said.
The new work is "fascinating, and it's kind of another piece of a puzzle" that needs much more work to define just what happens in the brain to turn on labor, he said.
The paraventricular nuclei regulate secretion of adrenocorticotropic hormone, called ACTH. Earlier research suggested this hormone plays a key role in fetal signaling to the mother in sheep.
For the new study, the Cornell scientists destroyed the paraventricular nuclei in five sheep fetuses. The surgery was done while the fetuses were in the womb.
The mother sheep never went into labor, and neither they nor the fetuses showed normal changes in hormone levels that are associated with labor. After the normal gestation period, surgeons removed the fetuses, which had otherwise developed normally.
The results show that the paraventricular nuclei play a necessary role in triggering birth, the researchers said.
Nathanielsz said he believes they act like "a little computer," collecting status reports from the fetal body and then sending the hormonal signal to start labor.