"We often think of the diseases that secondhand smoke causes as diseases of older people," epidemiologist Andrew Hyland told Reuters Health. "The results of this study show that secondhand smoke can affect even unborn babies."
Hyland led the study at the Roswell Park Cancer Institute in Buffalo, New York. He and his colleagues found the pregnancy risks associated with women's secondhand smoke exposure were almost as high as the risks related to their own cigarette smoking.
The study was the first to investigate the effects of secondhand smoke using quantified, lifetime exposure levels. The analysis arms clinicians like Dr. Maurice Druzin, from Stanford University Medical Center in California, with facts to try to persuade expectant fathers and others living with pregnant women to refrain from smoking at home.
"This is excellent ammunition for us to emphasize what we've known for a long time, but now we've got data to support it," Druzin, who was not involved in the study, told Reuters Health.
"This is the first study that shows that secondhand smoke has the same effect as being a primary smoker," he said. "That is a game changer."
Hyland's team used data from a study of 80,762 women between the ages of 50 and 79 years old. Researchers asked the women about their own smoking and the amount of secondhand smoke they were exposed to as children and adults, as well as about their history of pregnancy problems.
Among women who never smoked themselves, the chances of having a stillbirth were 22 percent higher for those who were exposed to any tobacco smoke than for unexposed women. That was after the researchers took into account other potential contributors, including women's weight, education and alcohol drinking.
For women who were exposed to the highest lifelong levels of secondhand smoke, the risk of having a stillbirth was even greater - 55 percent higher than among unexposed women.
The researchers defined the highest level of exposure to secondhand smoke as at least 10 years of exposure during childhood, at least 20 years during adulthood and at least 10 years in the workplace.
At that level, a woman's risk of a tubal ectopic pregnancy was 61 percent higher than among unexposed women, and her risk of a miscarriage was 17 percent higher.
"We're not talking about an elevated risk of a rare event," Hyland said of the miscarriage finding. "We're talking about something that happens all the time."
Image: Man smoking near pregnant woman, via Shutterstock