Women exposed to secondhand smoke while pregnant are more likely to experience a stillbirth or have babies with birth defects, a new meta-analysis concludes.
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 online today in Paediatrics.
"Women need to be protected from passive smoke exposure before conception and throughout pregnancy," Dr Jo Leonardi-Bee, a professor at the University of Nottingham in England and one of the authors of the study, said.
Although the increased risks of stillbirth and birth defects are not massive, Dr Leonardi-Bee wrote, "they are a lot larger in magnitude than what one would anticipate if we believe that passive smoke only has 1% of the effect of active smoking."
Dr Leonardi-Bee and her colleagues combined data from 19 studies that looked at the effects of secondhand smoke on the rates of miscarriage, neonatal death, and birth defects.
The rates of miscarriage and newborn death were similar whether or not women were exposed to secondhand smoke.
When looked at individually, no single birth defect 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 second hand smoke from colleagues or family members.
Fathers the main culprit
In half of the studies analysed, fathers were the primary source of secondhand smoke.
Dr Charletta Ayers, who was not involved in this study, said the research confirms what many doctors have assumed about the risks of secondhand smoke.
"It's good now to have evidence to share with moms and families, especially with fathers," Dr Ayers, a professor at Robert Wood Johnson Medical School in Piscataway, New Jersey, said.
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 said.
The findings don't prove that tobacco smoke 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 behaviour could be involved. The study did not look at whether the pregnant women had smoked in the past.
Dr Leonardi-Bee said another 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.
(Reuters Health, Kerry Grens, March 2011)