Pregnant women who have been exposed to high levels of secondhand smoke have a higher rate of miscarriages, stillbirths and fetal deaths, a new study suggests.
"We often think of the diseases that secondhand smoke causes as diseases of older people," said epidemiologist Andrew Hyland. "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," said Druzin, who was not involved in the study.
"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."
Nearly one third of women in the study reported at least one miscarriage, 4.4 percent reported at least one stillbirth and 2.5 percent reported at least one tubal ectopic pregnancy, according to findings published in Tobacco Control.
Ectopic pregnancy occurs when a fertilized egg attaches outside the uterus, usually in one of the fallopian tubes. Tubal pregnancies are the leading cause of maternal death during the first trimester of pregnancy, according to the Centers for Disease Control and Prevention.
Researchers cannot draw firm conclusions about cause and effect from observational studies, like the current one. But the study results point to the benefits of minimizing exposure to secondhand smoke, Hyland said.
"There's a biological plausibility that secondhand smoke could have an impact on these reproductive outcomes not only during the reproductive years but throughout the lifetime of a woman," he said.
"The take-home message is these never-smoking women who had the highest levels of exposure to secondhand smoke had the highest risks," he said. "These risks were generally comparable to the risks for women who ever actively smoked."
Prior research firmly established that smoking during pregnancy is associated with a higher risk of fetal death, the authors write. Smoking during pregnancy also has been linked to infertility, premature birth, low birth weight, birth defects and sudden infant death syndrome, they add.
The American Cancer Society estimates that 10 to 15 percent of women smoke during pregnancy and that as many as 5 percent of infant deaths could be prevented if pregnant women did not smoke.
Tobacco researcher Stanton Glantz said the current study adds to a growing body of research suggesting that secondhand smoke is linked to nearly as many health problems as active smoking.
"The interesting thing is that the passive smoking risks weren't terribly different from the active smoking risks," said Glantz, from the University of California, San Francisco. He was not involved in the new study.
"Even secondhand smoke is giving you enough exposure to trigger these bad reproductive effects," he said. "If people want to smoke, they should go outdoors, away from other living things."