The proportion of women dependent on drugs such as narcotic painkillers or heroin during pregnancy has more than doubled in the past decade and a half, a new study finds, though it still remains below a half-percent of all pregnancies.

The study covers a class of drugs known as opioids, which include prescription painkillers such as oxycodone (Oxycontin) and hydrocodone bicarbonate (Vicodin), as well as illegal drugs such as heroin.

Dependence on these drugs during pregnancy is linked to several increased risks during delivery, even when compared to women abusing or dependent on non-opiate drugs, explained study senior author Dr. Lisa Leffert, chief of the Obstetric Anesthesia Division at Massachusetts General Hospital in Boston.

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