Dr. Jo Leonardi-Bee of the Center for Tobacco Control Studies at the University of Nottingham says the findings were drawn from a systematic review of 19 studies from North America, South America, Asia and Europe. The studies centered on pregnant women who did not smoke but were exposed to secondhand smoke at home or at work.
None of the studies found an increased risk of miscarriage or newborn death from secondhand smoke and passive smoking was not associated with any one congenital defect. The overall increase was only seen after the results from all the studies were pooled, Leonardi-Bee says.
"What we still don't know is whether it is the effect of sidestream smoke that the woman inhales that increases these particular risks or whether it is the direct effect of mainstream smoke that the father inhales during smoking that affects sperm development, or possibly both," Leonardi-Bee says in a statement.
The study, scheduled to be published in the April edition of the journal Pediatrics, found passive smoking increased the risk of still birth by 23 percent and was linked to a 13 percent increased risk of congenital birth defects.