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More doctors are prescribing opioid painkillers to pregnant women, the New York Times reported.

In a study published in Obstetrics and Gynecology, data revealed that of the 1.1 million pregnant women enrolled in Medicaid nationwide, nearly 23 percent filled a prescription for opioids in 2007, compared to 18.5 percent in 2000.

“One in five women using opioids during pregnancy is definitely surprising,” study author Rishi J. Desai, a research fellow at Brigham and Women’s Hospital in Boston, told the New York Times.

Another study, published in the journal Anesthesiology, revealed that among a cohort of 500,000 privately-insured women, 14 percent received opioid painkillers at least once during pregnancy.

In both studies, codeine and hydrocodone were the most commonly prescribed painkillers and only 2 percent of women reported for taking them for longer than a week. Rates of opioid prescriptions among pregnant women were highest in the South and lowest in the Northwest and Northeast, the New York Times reported.

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Risks associated with exposing a developing fetus to opioids are unknown, though some evidence has linked exposure to the drug to birth defects.

“Opioid use in very early pregnancy is associated with an approximate doubling the risk of neural tube defects,” said Martha M. Werler, a professor of epidemiology at the Boston University School of Public Health. “About half of pregnancies are not planned, so that’s a big chunk of women who may not be thinking about possible risks associated with their behavior.”

Another study published in JAMA in 2012 suggested more babies are also being born addicted to opioids.

Researchers said it is unclear why opioids have surged in popularity among pregnant women, though they suggested that some may view painkillers as an easier solution to problems such as back pain, compared to physical therapy.

The U.S. Centers for Disease Control and Prevention (CDC) recently launched the Treating for Two initiative, which aims to keep expecting parents and physicians up-to-date on how to safely use medications during pregnancy.

Click for more from the New York Times.