"Through medical imaging examinations, we are exposing pregnant women to twice the amount of radiation as we did 10 years ago," said Elizabeth Lazarus, a doctor at Brown University in Providence, Rhode Island, in a report introducing the study.
"Overall, the levels of radiation to which we are exposing pregnant women are low, but they do carry a slight risk of harm to the developing fetus," said Lazarus, lead author of the study presented Tuesday at the annual meeting of the Radiological Society of North America.
Researchers reviewed different types of imaging examinations carried out over 10 years at the university.
They found that from 1997 to 2006, the number of imaging exams on pregnant women increased 121 percent, while the number of births rose by only seven percent.
The greatest increase was in the number of CT (computerized tomography) scans, "which deliver more radiation than many other radiologic procedures," the report said. CT scans increased by a quarter.
An ultrasound scan, on the other hand, does not expose the mother and fetus to the kind of radiation that can harm cells.
Lazarus said the increase was partly due to the development of new imaging techniques for diagnosing abnormalities -- and partly because hospitals and insurers wanted to make faster diagnoses.
A pregnant woman may have a CT scan to detect suspected life-threatening conditions such as bleeding in the brain, blood clots in the lungs or appendicitis.
"CT can be a safe, effective test for pregnant patients," Lazarus said. "However, there are alternatives that should at least be explored. Pregnant patients should ask their doctors about other imaging or diagnostic tests that may not expose the fetus to radiation."