People with sleep apnea experience repeated, brief interruptions in their breathing during sleep.
The University of South Florida study authors also found that sleep apnea is associated with higher rates of serious medical conditions that are the leading causes of pregnancy- and childbirth-related death, including an enlarged heart, blood clots in the lungs, and the dangerous blood pressure conditions known as preeclampsia and eclampsia.
The team analyzed data from an estimated 55 million women who were pregnant or gave birth in U.S. hospitals between 1998 and 2009. The study was published online May 1 in the journal Sleep.
"The astounding association with maternal death was surprising," lead author Dr. Judette Louis, assistant professor of obstetrics and gynecology, said in a university news release. "I did not expect to find such a difference in mortality between pregnant women who had sleep apnea and those who did not, especially when we controlled for obesity and other complicating factors."
The increased risk of death may be due to the added stress that sleep apnea places on the normal physical demands of pregnancy, she said.
However, although the study found an association between sleep apnea and an increased risk of pregnancy-related death, it did not prove a cause-and-effect relationship.
The researchers noted that maternal death rates in the United States have increased in recent years, and a rising obesity rate is one suspected cause.
"Our study indicates that sleep apnea may also play a role, whether a woman is obese or not," Louis said. "It's important for obstetricians and primary care practitioners to identify sleep apnea in younger women of reproductive age, convey its risk, and treat the condition before pregnancy."
Previous, smaller studies have found that sleep apnea increases the risk for problems during pregnancy, but this is the first large-scale analysis linking sleep apnea with increased risk of maternal death.
The U.S. Office of Disease Prevention and Health Promotion offers tips for a healthy pregnancy.