Despite the risks associated with their use, doctors have been prescribing prescription opioid painkillers increasingly over the past few years. In 2012 alone, physicians wrote 259 million prescriptions for these painkillers, which are often abused. While overdoses are an obvious concern, a new study highlights another risk associated with their excessive use: Too many women of childbearing age are being prescribed the drugs.

The study, published in the Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report, found over a third of women capable of bearing children who were enrolled in medicaid and over a quarter enrolled in private insurance were filled opioid painkiller prescriptions each year between 2008 and 2012. The most common opioids prescribed were codeine, oxycodone, and hydrocodone, all of which are typically used for pain or as additives in cough syrup.

“Taking opioid medications early in pregnancy can cause birth defects and serious problems for the infant and the mother,” CDC Director Dr. Tom Frieden said in a press release. “Many women of reproductive age are taking these medicines and may not know they are pregnant, and therefore may be unknowingly exposing their child.”

Opioid painkillers have been linked to severe birth defects like those of the neural tube, as well as neonatal abstinence syndrome (NAS). Neural tube defects emerge during the first month of pregnancy, and result in developmental problems in the fetus’ brain or spine — anencephaly, one of these defects, occurs when the brain hasn’t fully developed. Meanwhile, a mother who uses opioid drugs while pregnant is likely to give birth to a child dependent on the drugs. NAS occurs when they’re no longer exposed to the drugs and they start experiencing symptoms of withdrawal.

For the study, the researchers looked at two large data sets of insurance claims from 2008 to 2012 comprising women aged 15 to 44. They found 39 percent of medicaid enrollees and 28 percent of privately insured women filled opioid prescriptions from pharmacies during each year.

In the paper, the researchers wrote that “the consistently higher frequency of opioid prescribing to Medicaid-enrolled women is of concern because approximately 50 percent of U.S. births occur to Medicaid-enrolled women.” That could translate into a lot of birth defects, yet it’s unclear why so many are taking the opioids. The researchers speculated it could be due to differences in prescription drugs covered under their insurance plans, differences in the way they obtained health care services, or differences in prevalence of underlying conditions.

With this study, Frieden and the CDC hope to bring light to the need for doctors to prescribe opioid drugs responsibly, and to discuss the risks and benefits of using the powerful drugs.

Source: Ailes E, Dawson A, Lind J, et al. Opioid Prescription Claims Among Women of Reproductive Age — United States, 2008–2012. Morbidity and Mortality Weekly Report. 2015.