Working nights while pregnant increases the risk of giving birth prematurely by up to 50%, a study has suggested.
Night shifts may affect what should happen in the uterus
University of North Carolina researchers looked at the working conditions of 1,900 pregnant women.
The Obstetrics and Gynaecology study found standing for long periods and lifting heavy weights did not increase the risk of premature labour.
But working nightshifts in the first three months was linked to a doubling in a woman's risk of early labour.
The researchers say this is possibly because they disrupt the normal activity in the womb at night.
The women who took part in the study were all interviewed in the seventh month of their pregnancy.
The women were asked to report details about their jobs, such as how many hours per day they spent standing, and how many times per day they lifted an object that weighed 25 pounds or more.
Relatively few women regularly performed heavy lifting - 10% during the first trimester, and about 6% later in pregnancy.
About one-quarter spent most of the day standing while they were in the first trimester, and roughly 20% did so during the second trimester and seventh month.
The researchers found that even women who spent more than 30 hours a week on their feet were no more likely than other women to give birth prematurely, or to have a smaller than normal baby.
The same was true of women who repeatedly lifted heavy objects, even in they did so more than 13 times per week.
But the 9.2% of women (166) who worked nights were found to be at a 50% increased risk of giving birth early.
However, the researchers say the reason for the link is unclear, and the stress that because relatively few women in the study actually worked nights, particularly as their pregnancy progressed.
Body clock 'disruption'
Dr Lisa Pompeii, who led the research, told the BBC News website: "The findings from our study are based on a small sample size and need to be interpreted with caution.
"However, these findings, along with a handful of other studies that have observed a modest elevation in the risk of preterm delivery among women who work at night and/or shift work during pregnancy, warrants the need for further exploration of the biological effects of shift work on uterine activity during pregnancy.
"Further studies would help clarify the possibility of a causal effect of shift work on preterm birth."
She added: "Our findings suggest that women who work at night during pregnancy may be at an elevated risk for preterm birth.
"Based on our study findings, it would be premature to make recommendations to pregnant women about night work.
"However, further studies need to be done to explore whether or how shift work influences uterine activity during pregnancy."
Richard Warren, of the Royal College of Obstetrics and Gynaecology, said: "This paper is interesting because it adds to previous debates and the uncertainty as to whether exertive physical activity is a risk in pregnancy. "
But he added: "The effects of night work needs more evaluation to confirm and establish any risk, and whether or not this is true throughout pregnancy, particularly as the numbers looked at in this study are small."