Prescription painkillers are used widely by U.S. women of childbearing age, a federal report released on Thursday found, and health officials said exposure to such drugs during pregnancy could increase the risk of birth defects.

Of women aged 15-44, more than a third on Medicaid and a fourth on private insurance filled prescriptions for so-called opioid pain medications each year between 2008-2012, according to the Centers for Disease Control and Prevention.

Opioids include medications such as hydrocodone, codeine and oxycodone that are taken to treat moderate to severe pain. The study was the first by the CDC to specifically examine their use by women of reproductive age.

Taking the drugs during pregnancy, particularly in the early weeks, can increase the chances that babies will be born with birth defects, the CDC said.

Given the popularity of the painkillers, a thorough health assessment of women of reproductive age is crucial before they are prescribed, officials said.

"Many women of reproductive age are taking these medicines and may not know they are pregnant and therefore may be unknowingly exposing their unborn child,” CDC Director Tom Frieden said in a statement.

Prescription rates of opioids for reproductive-aged women were highest in the South and lowest in the Northeast, the CDC study said.

The study did not address why more patients on Medicaid, the federal government’s health insurance plan for the poor, use prescription painkillers. One reason could be that they have more health problems that require pain medication, said CDC epidemiologist Jennifer Lind, one of the study's authors.

“There may be more women on Medicaid who are manual laborers, jobs that cause more chronic pain,” Lind said in an interview.

Half of all births in the United States are to mothers on Medicaid, according to the CDC.

The federal health agency has launched a program called “Treating for Two” to increase awareness on the potential dangers of prescription painkillers to women of child-bearing age, Lind said.

“What we’re trying to do is expand research and also develop reliable guidance so that women and healthcare providers can have conversations and make informed decisions on what the safest options may be,” she said.