The percentage of women dependent on opioids during pregnancy more than doubled from 1998 to 2011, a new study finds. The overall rate of opioid dependency in pregnant women remains low, at 0.39 percent.

The study looked at the use of prescription painkillers such as oxycodone, hydrocodone, morphine and methadone, as well as illegal opioids such as heroin, according to HealthDay.

Lead researcher Dr. Lisa Leffert of Massachusetts General Hospital in Boston noted dependence on opioids during pregnancy can increase risks during delivery. “This increase in opioid abuse and dependence in the pregnant population is happening along with that in the general population,” she said. “These women were more likely to deliver by cesarean and have extended hospital stays” compared to other pregnant women.

Leffert and colleagues analyzed data on almost 57 million deliveries between 1998 and 2011. They found women who abused or were dependent on opioids were almost five times as likely to die during hospitalization, and were more likely to deliver by cesarean section and have extended hospital stays, compared with women who did not abuse or were not dependent on narcotics.

The babies of women who abused or were dependent on opioids were twice as likely to be stillborn, premature and have poor growth. A condition called placental abruption, in which the placenta separates from the wall of the uterus prematurely, was almost three times as likely in these women.

The findings are published in the journal Anesthesiology.