Pregnant women who work the night shift may be more likely than those with traditional work hours to deliver prematurely, study findings suggest.

On the other hand - and contrary to some past research - the study also found that physical demands on the job - including standing for most of the day or lifting heavy objects - were not related to premature delivery or having a smaller-than-normal baby.

Dr. Lisa A. Pompeii and colleagues at the University of North Carolina, Chapel Hill report the findings in the journal Obstetrics & Gynecology.

Previous studies into the pregnancy effects of on-the-job exertion have yielded conflicting results, according to the researchers. One reason, they note, may rest in differences in the ways the studies have defined and measured “exertion.”

For their study, Pompeii and her colleagues used data on more than 1,900 pregnant women in North Carolina who were interviewed about their work conditions through the seventh month of pregnancy.

The women reported, among other things, how many hours per day they spent standing, and how many times per day they lifted an object that weighed 25 pounds or more.

The researchers found that women who spent many hours on their feet - more than 30 per week - were no more likely than their peers to have a premature delivery or a smaller-than-average newborn. The same was true of women who repeatedly lifted heavy objects, even in excess of 13 times per week.

Relatively few women regularly performed heavy lifting - 10 percent during the first trimester, and about 6 percent later in pregnancy. About one-quarter spent most of the day standing while they were in the first trimester, and roughly 20 percent did so during the second trimester and seventh month.

But while physical exertion did not appear to promote early delivery, working the night shift did. Women who, at any point in pregnancy, worked between the hours of 10 p.m. and 7 a.m. were more likely than daytime workers to have a premature infant.

Those who worked the night shift during the first trimester, for example, were 50 percent more likely to deliver prematurely than their peers who worked day jobs.

The reason for the link is unclear, and the researchers stress that because relatively few women in the study worked the night shift - particularly in the seventh month of pregnancy - “it is necessary to interpret these findings with caution.”

There is some evidence that during pregnancy, activity in the uterus changes at night. It’s possible, the study authors speculate, that because night work disrupts the body “clock,” it affects normal activity of the uterus.

Other studies, the researchers note, have found that night work may suppress the hormone melatonin, which helps regulate the body’s normal functional rhythms. More research, they conclude, is needed to see whether melatonin is important in uterine activity during pregnancy, and whether changes in melatonin due to shift work affect pregnancy outcomes.

SOURCE: Obstetrics & Gynecology, December 2005.