The rate of induced labor among U.S. pregnant women nearly doubled between 1992 and 2003 - a trend that seems to have pushed more births to the earlier end of full-term, a new study finds.

The pattern raises concerns, researchers say, because even though babies born at the 37th or 38th week of pregnancy are considered full-term, they do have a relatively higher risk of problems compared with infants born later.

Typically, pregnancy lasts about 40 weeks, and any birth between the 37th and 41st weeks is considered full-term. (Pregnancies lasting 42 weeks or longer are considered “post-term.”)

In the new study, researchers used U.S. government vital statistics to show that labor inductions increased from 14 percent of full-term singleton births in 1992, to 27 percent in 2003.

Moreover, those same years saw an increase in the proportion of births at the earlier end of full-term pregnancy. In 2003, 30 percent of births occurred during the 37th or 38th week, versus 19 percent in 1992. And just over 60 percent of full-term births occurred before the 40th week - up from 42 percent in 1992.

The findings suggest that the rising rate of induced labor is a “likely cause” of the earlier births, the researchers report in the American Journal of Obstetrics & Gynecology.

The concern is that even though newborn deaths and other complications are uncommon at weeks 37 or 38, the risks are relatively higher compared with infants born at the later end of full-term, explained senior researcher Dr. Michael S. Kramer, of McGill University in Montreal, Canada.

Rates of death and problems such as respiratory distress are higher during those earlier weeks, and research suggests that fetal brain development can be affected - with babies born relatively earlier showing modestly lower IQ scores, in general, later on.

The absolute risk of such problems for any one woman having a labor induction is quite low, Kramer stressed. And women should not be discouraged from having a labor induction if it is medically necessary, he said.

However, Kramer said, both obstetricians and women should be aware that labor induction at the 37th or 38th week is not without risks. And studies should continue to examine the potential public-health impact of the rising labor induction rate.

“I’m not saying we’re at the point that we’re doing too many labor inductions,” Kramer said. But if the rate continues to increase, he added, “eventually, we may get to the point where we do more harm than good.”

Precisely why labor inductions rose between 1992 and 2003 is not known; Kramer’s team had no information on the medical reasons for individual women’s inductions.

In general, labor inductions are often performed when a pregnancy goes past 41 weeks, because the risks of complications for mother and baby rise after that point.

For women who are not yet post-term, inductions may be done under circumstances where the fetus is in danger of not getting enough nutrients or oxygen from the placenta; when a woman’s “water breaks” but labor does not begin; or in some cases in which the mother has a medical condition - like gestational diabetes or high blood pressure - that is putting her health or her baby’s health at risk.

Kramer said that labor inductions may be on the rise because doctors are more closely monitoring pregnancies, aided by newer technologies like Doppler ultrasound. That, he said, is giving doctors and expectant parents more information - which includes “more information that can make them worry.”

So labor inductions may be increasingly performed in cases where there are concerns, but it is not clear that induction is necessary - such as when a fetus is growing, but not quite at the rate the doctor and parents would like.

Kramer said he thinks that large clinical trials should be designed to look at the effects of labor induction versus “watchful waiting” in such cases where the necessity of induction is not clear.

It is also possible, according to Kramer, that some labor inductions are being done for the sake of “convenience” - for the doctor or parents - but he said that this practice would be fairly uncommon and unlikely to account for many of the inductions seen in this study.

SOURCE:

American Journal of Obstetrics & Gynecology, online May 17, 2010.