Pregnant women who live or work with smokers may be at slightly higher risk of having a stillbirth, suggests a study that adds to evidence that even second-hand smoke can harm unborn babies.
Newborns also weighed a little less and had smaller heads if their mothers were passive smokers, Canadian researchers found.
"This information is important for women, their families and healthcare providers," Dr Joan Crane of Eastern Health in St. John's and colleagues write in the BJOG: An International Journal of Obstetrics and Gynaecology.
Second-hand smoke is thought to expose people to about 1% of the smoke that active smokers inhale. According to the researchers, "undiluted side-stream smoke contains many harmful chemicals and in greater concentration than cigarette smoke inhaled through a filter".
Those chemicals may harm the foetus in a variety of ways, for instance by restricting blood flow and possibly damaging the placenta.
Little is known about the risk of stillbirth in passive smokers, so Crane and her colleagues used a database of pregnant women from the Canadian provinces of Newfoundland and Labrador to shed light on the question.
They also looked at other birth outcomes, such as head circumference, which has been linked to kids' intellectual development.
Of nearly 12,000 women in the database, 11 % said they had been exposed to second-hand smoke.
The rate of stillbirth, in which the baby dies during the third trimester of pregnancy, was 0.83 % in passive smokers and 0.37% in women who didn't breathe tobacco fumes.
That doesn't prove that smoke itself was the culprit, because other risk factors might be different between the two groups.
Yet when the researchers accounted for several of those, including age and the women's drinking and drug habits, passive smokers had more than three times the odds of stillbirth.
In other words, if smoke is indeed to blame, one extra baby would die in the womb for every 117 women exposed.
"This is huge," said Dr Hamisu Salihu, an expert on stillbirth at the University of South Florida in Tampa. "We can now inform patients that exposure to second-hand smoke means they can lose their baby."
That link had not been firmly established until now, Salihu, who was not involved in the work, said.
Common causes of stillbirth
On a global scale, the most common causes of stillbirth are complications during childbirth, infections like syphilis during pregnancy, health problems like high blood pressure or diabetes, foetal growth restriction - in which babies fail to grow at the proper rate - and birth defects.
Most miscarriages, on the other hand, happen in the first trimester and most are believed to be due to random genetic abnormalities.
Still, certain lifestyle habits have been tied to a higher risk of miscarriage, including heavy drinking, drug use and, in some studies, smoking.
The Canadian researchers also found that babies born to passive smokers weighed 54 grams, or nearly two ounces, less than babies whose mothers lived and worked in smoke-free households.
And their heads were slightly smaller, too, measuring 0.24 centimetres (about 0.1 inch) less on average.
Salihu said head circumference has been associated with IQ, although the link is indirect.
"Policy makers should really take this matter seriously," he concluded. "We need to enact laws to protect these babies." (Reuters Health/May 2011)
Smoking doubles risk of stillbirth