Researchers at the University of Adelaide in Australia analyzed data gathered from more than 170,000 women over 10 years in what is described as the first study of its kind.
"We know that being pregnant and having asthma poses risks to both the mother and the baby. We know that smoking poses risks to both the mother and the baby. But now we also know that the combination of these conditions represents a very dangerous situation," study author Dr. Nicolette Hodyl said in a university news release.
"Asthma and smoking are separately linked during pregnancy to increased risk of bleeding from the birth canal before labor, urinary tract infections, premature rupture of membranes, low birth weight and preterm birth [less than 37 weeks of pregnancy]," she noted. "The combination of asthma and smoking greatly increases the risk of these complications during pregnancy."
Rates of preterm birth were 5.8 percent for women who did not have asthma and did not smoke, 6.5 percent for those with asthma, 9.4 percent for smokers, and 12.7 percent for those who had asthma and also smoked, the study found.
"This is an alarming statistic. We hope that pregnant women begin to understand the seriousness of this situation to their health and the health of their child," Hodyl said.
The researchers also found that about one-quarter of pregnant women with asthma are smokers.
"While the rates of smoking have been decreasing in recent years, it is very concerning to us that many pregnant women with asthma are also smoking," Hodyl said.
The study was published online ahead of print in the European Respiratory Journal.
"Quitting smoking during pregnancy is very difficult, and therefore pregnant women need as much support as possible from family, friends and health professionals," Hodyl said. "Our results show that even a reduction in the number of cigarettes women smoke per day can lead to some improvement to the risks to their child. However, the potential for poor health outcomes for both the mother and child should not be underestimated."
While the study showed an association between smoking, having asthma and an increased risk of pregnancy complications, it did not prove a cause-and-effect link.
The Nemours Foundation outlines how to stay healthy during pregnancy.