The federal health authorities reported Thursday that nearly one-third of women of reproductive age had had an opioid painkiller prescription filled every year from 2008 to 2012. Experts said the practice carried considerable risks for birth defects.

The Centers for Disease Control and Prevention analyzed health insurance claims data from Medicaid and private insurers for women ages 15 to 44 and found that an average of 39 percent of women on Medicaid filled an opioid prescription in a pharmacy each year from 2008 to 2012, compared with 28 percent of women with private insurance.

Dr. Thomas R. Frieden, director of the C.D.C., described the numbers as “astonishing” and said they presented a substantial risk for birth defects. Women often do not know they are pregnant in the early weeks after conception, a critical time for organ formation, and could be “unknowingly exposing their unborn child” while taking the drugs.

Exposure to opioid painkillers increases the risks for major defects in the baby’s brain and spine, congenital heart defects and problems with the baby’s abdominal wall.