NEW YORK (Reuters) Some studies have suggested that working the night shift may raise a pregnant woman's risks of preterm delivery or having an underweight baby, but a review says that if those effects exist, they are likely to be small.After looking at 23 studies involving thousands of women, researchers led by Matteo Bonzini of the University of Insubria in Italy found that overall, shift work was not strongly linked to the risk of preterm delivery versus a standard nine-to-five job.Women working night or rotating shifts did have a slightly higher chance of having a baby who was small for gestational age, but the evidence was not strong enough to make "confident conclusions”, the researchers reported in the journal BJOG."On balance, the evidence currently available about the investigated birth outcomes does not make a compelling case for mandatory restrictions on shift-working in pregnancy," they wrote.In theory, irregular work hours could affect a woman's reproductive function by throwing off the body's natural clock and disrupting normal hormone activity. A recent US 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 raising the possibility that rotating shifts might affect fertility.Whether that is the case, though, is unknown. Studies have also yielded conflicting findings about whether women on night and rotating shifts have higher risks of preterm delivery or having an underweight baby.However, many factors would 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.Some studies factored in many of those variables, but others did not. The review included 23 international studies, each involving anywhere from 700 to more than 35,000 women.When the researchers combined the results from all the studies looking at preterm delivery, there was a slightly higher risk seen among shift workers 16 percent compared to non-shift workers.But after the researchers sifted out several studies they deemed low quality, either 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 labour disappeared.As for birth size, there were somewhat higher risks seen among women doing shift work they were 12 percent more likely, for example, 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 said.They added that there is a need for further studies."In the meantime, 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," they wrote.