Researchers have found that nonsmoking pregnant women exposed to secondhand smoke at home or at work are significantly more likely to have a stillborn birth or a child with a birth defect — and suggest men quit smoking before trying to have a baby — the BBC reported March 11.

Researchers at the University of Nottingham in the U.K. reviewed 19 studies to determine whether passive smoking affected the rates at which nonsmoking pregnant women experienced “spontaneous abortion, perinatal and neonatal death, stillbirth, and congenital malformations,” according to the study abstract.

They found that women exposed to 10 or more cigarettes a day were 23 percent more likely to have a stillborn birth, and 13 percent more likely to have a baby with a congenital malformation. The risks of miscarriage or death of the newborn did not increase.

“It is vital that women are made aware of the possible risks associated with second-hand smoke and alert those around them of the impact it could potentially have on the health of their unborn baby,” said Andrew Shennan, a professor of obstetrics at St. Thomas' Hospital in London.

What remained unclear was which mattered more: exposure to secondhand smoke, or the impact of cigarette smoking on the father's sperm.

“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,” said Dr Jo Leonardi-Bee of the University of Nottingham, who led the study.

“More research is needed into this issue although we already know that smoking does have an impact on sperm development, so it is very important that men quit smoking before trying for a baby.”

The study, “Secondhand Smoke and Adverse Fetal Outcomes in Nonsmoking Pregnant Women: A Meta-analysis,” was published online in Pediatrics on March 7, 2011.