More than one-fourth of women who might become pregnant are getting prescriptions for opiod painkillers, such as Vicodin and Oxycontin, that can cause birth defects and other serious problems early in pregnancy, according to a report released Thursday by the Centers for Disease Control and Prevention.

These common opiod medications are typically prescribed to treat moderate to severe pain, and they can also be found in some prescription cough medicines. But taking them early in pregnancy is dangerous.

"They're deadly, they're addictive, and they cause birth defects," said CDC Director Tom Frieden.

There has been widespread attention to overdoses from prescription opioids. Previous studies of opiod use during pregnancy suggest the medicines could increase risk of major defects of the baby's brain and spine, heart and abdominal wall.

But this is the first time that CDC has looked into opioid painkillers specifically among women of child-bearing age, which is important because many pregnancies aren't recognized until well after the first few weeks, and half of all U.S. pregnancies are unplanned, officials said.

CDC researchers analyzed data from 2008 to 2012 from two sets of data for women age 15 to 44: those with private insurance and those enrolled in Medicaid. They found that, on average, 39 percent of Medicaid-enrolled women filled at least one opiod prescription each year compared to 28 percent of women with private insurance.

Among the reasons for the different prescribing rates may be differences in coverage under their health plans and differences in the women's underlying health conditions, the report said.

Geographic data available from the private insurance claims showed opiod prescription rates were highest for women in the South and lowest in the Northeast. Race and ethnicity data from Medicaid showed opiod prescriptions were nearly one and a half times higher for white women who might become pregnant, compared to black or Hispanic women.