NEW YORK (Reuters Health) - Women exposed to secondhand smoke while pregnant are more likely to experience a stillbirth or have babies with birth defects, a new study concludes.
Stillbirth, in which the fetus dies during the second half of the pregnancy, was 23 percent more common and birth defects were 13 percent more common among women who lived or worked with smokers.
"Women need to be protected from passive smoke exposure before conception and throughout pregnancy," Jo Leonardi-Bee, a professor at the University of Nottingham in England and one of the authors of the study, wrote in an email to Reuters Health.
According to the Centers for Disease Control and Prevention, birth defects occur in one out of every 33 births. Stillbirth is considerably less common, occurring in fewer than one in 100 births.
Secondhand smoke is thought to expose people to about one percent of the smoke that active smokers inhale.
Although the increased risks of stillbirth and birth defects are not massive, Leonardi-Bee wrote, "they are a lot larger in magnitude than what 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 secondhand smoke on the rates of miscarriage, death of an unborn or newborn baby, and birth defects.
The rates of miscarriage and newborn death were similar between women who breathed secondhand smoke and those who didn't.
When looked at individually, no single birth defect -- such as a heart abnormality or misshapen body part -- was linked to secondhand smoke. Only when the researchers pooled the data on all birth defects did they see an increased risk.
None of the women in the studies smoked while pregnant, but they breathed in secondhand smoke from colleagues or family members.
In half of the studies analyzed, fathers were the primary source of secondhand smoke.
Dr. Charletta Ayers, who was not involved in this study, said the research confirms what doctors assumed about the risks of secondhand smoke.
"It's good now to have evidence to share with moms and families, especially with fathers," Ayers, a professor at Robert Wood Johnson Medical School in Piscataway, New Jersey, told Reuters Health.
Stephen Grant of Magee-Womens Research Institute in Pittsburgh, who wasn't involved in the study either, said he's most intrigued by the association between secondhand 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," Grant told Reuters Health.
Still, the findings can't prove that tobacco smoke in itself causes birth defects or stillbirths. And even if it does, it's uncertain whether it's due to the mother inhaling the father's secondhand smoke or if his smoking is affecting his sperm.
Grant said there's also a possibility that a woman's previous smoking behavior could be involved. The study did not look at whether the pregnant women had smoked in the past.
A previous study by Leonardi-Bee and her colleagues found that secondhand smoke is tied to lower birth weight. And active smoking by pregnant mothers is associated with a range of problems, including up to a 40 percent increase in birth defects (see Reuters Health report, March 3, 2011).
She said one potential pitfall of her current study is that research with negative results is sometimes not published, which means studies that did not find an association between secondhand smoke and pregnancy problems may not have been included in her review.
The study, published in the latest issue of the journal Pediatrics, was funded by the UK Center for Tobacco Control Studies, which receives funds from government and non-profit sources, and by the UK Department of Health.
SOURCE: bit.ly/hcHotI Pediatrics, online March 7, 2011.