Some studies have suggested that working the night shift may raise a pregnant woman's risks of preterm labor or having an underweight baby. But a new review says that if those effects exist, they are likely to be small.
Looking at 23 studies involving thousands of women, researchers found that, overall, shift work was not strongly linked to the risk of preterm labor, versus the standard 9-to-5 job.
Women working night or rotating shifts did have a slightly higher chance of having a baby who was underweight or "small for gestational age" - meaning small for the baby's sex and the week of pregnancy during which he or she was born.
Still, the evidence was not strong enough to make "confident conclusions," the researchers report in the obstetrics journal BJOG.
Together, the results suggest that any effects of shift work on preterm delivery and birth size "are likely to be small," write the investigators, led by Dr. Matteo Bonzini of the University of Insubria in Italy.
In theory, irregular work hours could affect a woman's reproductive function by throwing off the body's natural "clock" and disrupting normal hormonal activity.
A recent U.S. government study, for instance, found that nurses who worked rotating shifts were more likely to have irregular menstrual periods than those who worked a consistent schedule. That, the researchers said, raised the possibility that rotating shifts might affect fertility.
Whether that is the case, though, is unknown. And when it comes to pregnancy, studies have come to conflicting findings about whether women on night or rotating shifts have higher risks of preterm delivery or having an underweight baby.
One of the problems is that many factors could potentially explain a connection between shift work and poorer pregnancy outcomes.
Women who do shift work may make less money, have higher smoking rates or generally less healthy lifestyles than women with a standard work week. In some studies, researchers are able to factor in many of those variables; in others, they're not.
The current review included 23 international studies, each involving anywhere from 700 to more than 35,000 women.
When the researchers combined the results from all studies looking at preterm delivery, there was a slightly higher risk seen among shift workers -- 16 percent higher, versus non-shift workers.
But then the researchers sifted out several studies they deemed low-quality - because they didn't account for smoking and income, or relied on women's self-reports rather than medical records.
Without those studies, the link between shift work and preterm labor disappeared.
As for birth size, there were somewhat higher risks seen among women doing shift work -- they were 12 percent more likely, for instance, to have a baby who was small for gestational age. But the evidence was not statistically strong, and the increased risk could be a chance finding, the researchers say.
"On balance," Bonzini and his colleagues write, "the evidence currently available about the investigated birth outcomes does not make a compelling case for mandatory restrictions on shift-working in pregnancy."
Still, they add, there's a need for further studies.
"In the meantime," they write, "we suggest that, it would be prudent, insofar as job circumstances allow, to permit pregnant women who wish to do so, to reduce their exposure to shift and night working."
In general, experts suggest that, for the sake of overall health, shift workers try to catch up on sleep when they can, and pay extra attention to their diet and exercise habits -- both of which can suffer when working irregular hours.