Pregnant women who work physically demanding jobs, long work weeks or irregular hours may be at increased risk of delivering prematurely or having an underweight baby, a new study suggests.

Researchers found that among more than 1,100 pregnant women they followed, those who said their jobs required them to be very physically active were at increased risk of having a low-birthweight newborn.

Overall, 21 percent of these women delivered an underweight baby, compared with 14 percent of women with fewer physical demands at work, according to findings published in medical journal BJOG.

In addition to physical demands, long work hours, shift work and temporary contract work were all linked to either preterm delivery or low birthweight.

Women who worked 40-plus hours per week or worked a shift schedule had a heightened risk of delivering an underweight baby. Those doing temporary contract work, had a four-fold higher risk of preterm delivery than those with permanent jobs.

The study cannot weed out the reasons for the links, according to the researchers, led by Dr. Isabelle Niedhammer of the University College Dublin in Ireland.

In the case of temporary contract work, Niedhammer told Reuters Health, it may be that these types of jobs carry poorer work conditions, including higher levels of stress.

For its part, physically demanding work - including jobs that require lifting, carrying heavy loads or long periods of standing - has been linked to adverse effects on pregnancy in previous studies, Niedhammer pointed out.

In this study, women in low-skill jobs were more likely to perform shift work, have long worked weeks or work physically demanding jobs.

However, the researchers accounted for factors like women’s education and lifestyle habits such as smoking, and found that work conditions seemed to affect pregnancy outcomes independently of socioeconomic.

According to Niedhammer, the findings suggest that women should talk to their doctors about their jobs during their first prenatal care visit - and, if necessary and possible, make adjustments to their work conditions.

And because job conditions are potentially modifiable, she and her colleagues conclude, more studies should examine how specific conditions can affect the health of a pregnancy.

SOURCE: BJOG, online April 7, 2009.