Women exposed to second-hand smoke while pregnant are more likely to experience a stillbirth or have babies with birth defects.
Stillbirth was 23% more common and birth defects were 13% more common among women who lived or worked with smokers, according to a report published in Paediatrics.
"Women need to be protected from passive smoke exposure before conception
and throughout pregnancy," said Jo Leonardi-Bee, a professor at the University of Nottingham in England and one of the authors of the study, in an email to Reuters Health.
Although the increased risks of stillbirth and birth defects are not massive, she warned: "They are a lot larger in magnitude than one would anticipate if we believe that passive smoke only has one percent of the effect of active smoking."
Leonardi-Bee and her colleagues combined data from 19 studies that looked at the effects of second-hand smoke on the rates of miscarriage, newborn death and birth defects.
The rates of miscarriage and newborn death were similar whether or not women were exposed to second-hand smoke
, and when looked at individually, no single birth defect was linked to second-hand smoke. Only when the researchers pooled the data on all birth defects did they see an increased risk.
None of the women smoked while pregnant, but they breathed in second-hand smoke from colleagues or family members. In half the studies analysed, fathers were the primary source of second-hand smoke.
Other medical experts said that the research confirmed what many doctors have assumed about the risks of second-hand smoke, though the findings did not prove that tobacco smoke causes birth defects or stillbirths
Even if it did, it was unclear if this was due to the mother inhaling the father's second-hand smoke or if his smoking was affecting his sperm.
Stephen Grant of Magee-Womens Research Institute in Pittsburgh, Pennsylvania, who wasn't involved in the study, said he was most intrigued by the association between second-hand smoke and birth defects.
"What we have here is that it's possible all the chemicals in tobacco smoke could have some effect on development," he told Reuters Health. Has your baby been affected by second-hand smoke?