Increased Use of CT Scans During Pregnancy in US and Canada

Veronica Hackethal, MD

July 24, 2019

Physicians should think twice before ordering CT for women who are pregnant, a new study suggests.

During the past 2 decades, rates of exposure to ionizing radiation from medical imaging during pregnancy has increased dramatically in the United States and Canada, report Marilyn L. Kwan, PhD, from the Division of Research, Kaiser Permanente Northern California, in Oakland, and colleagues.

The large, comprehensive study, published online today in JAMA Network Open, is among the first to evaluate trends in the use of a range of medical imaging techniques during pregnancy.

Researchers found that rates of CT scanning increased almost fourfold in the United States and doubled in Canada from 1996 through 2016. However, they also found that the use of MRI, which does not deliver ionizing radiation, has recently surpassed use of CT in both countries. Thus, it appears that physicians may be starting to curb the use of imaging that exposes pregnant women to radiation in favor of other potentially safer methods.

Rates of medical imaging should be monitored during pregnancy, according to the authors.

"This study has given us a chance to look more closely at the use of advanced imaging in pregnancy. It's important to quantify exposure to ionizing radiation because it can cause cancer and birth defects and should be kept to a minimum, especially during pregnancy," Kwan said in a press release.

The risks posed by ionizing radiation depend on the amount of radiation and the gestational age at which the fetus is exposed. Risks are highest during early pregnancy, when fetal organs are forming.

"Most pregnant women get routine ultrasound to monitor fetal growth, which delivers no ionizing radiation," Diana L. Miglioretti, PhD, co–lead author, said in the press release. Miglioretti is a professor of biostatics at the University of California, Davis, and is senior investigator with the Kaiser Permanente Washington Health Research Institute in Seattle.

"But occasionally, doctors may want to use advanced imaging to detect or rule out a serious medical condition of the expectant mother, most often pulmonary embolism, brain trauma or aneurysm, or appendicitis," she added.

Pulmonary embolism ranks among the top causes of maternal death in developed countries. Physicians often use CT scans or ventilation/perfusion (V/Q) scans to diagnose pulmonary embolism. Most guidelines recommend V/Q scans over chest CT, because the former deliver less radiation. Both CT and nuclear medicine deliver higher doses of ionizing radiation than conventional x-rays.

High Rates of Exposure

To compare trends in the use of medical imaging during pregnancy, researchers conducted a retrospective cohort study in which they analyzed electronic health plan data from seven collaborators in the National Cancer Institute's Radiation-Induced Cancer Study. These included six integrated healthcare systems in the United States and the single-payer healthcare system in Ontario, Canada.

They analzyed data from 3,497,603 pregnancies and 2,211,789 women who gave birth at 24 weeks' gestational age or later between January 1996 and December 2016. Twenty-six percent of pregnancies were in the United States, 92% of women were aged 20 to 39 years, and 85% of births were full term.

Kwan and colleagues evaluated exposure to the following medical imaging techniques: CT, MRI, angiography, fluoroscopy, conventional radiography, and nuclear medicine. They excluded ultrasound from comparisons. They adjusted for study site, maternal age at the start of pregnancy, gestational age at birth, and year of birth for the newborn.

Overall, 5.3% of pregnant women in the US sites and 3.6% of pregnant women in Ontario were exposed to imaging with ionizing radiation; 0.8% of US women and 0.4% of women in Ontario underwent CT scanning. Study sites showed considerable variation in the use of imaging during pregnancy. For example, use of CT varied from 0.4% to 1.1%.

In the United States, overall use of CT during pregnancy increased 3.7-fold during the study period, rising from 2.0 per 1000 pregnancies in 1996 to 11.4 in 2007. After that, rates leveled off through 2010, then declined to 9.3 per 1000 pregnancies in 2016.

In contrast, rates of CT imaging during pregnancy increased over this time in Ontario. Overall, though, rates in Canada were 33% lower than in the United States. In addition, the increase was more gradual. Rates of CT use doubled during the study period, going from 2.0 per 1000 pregnancies in 1996 to 6.2 in 2016.

In both countries, MRI rates steadily increased, rising from 1.0 per 1000 pregnancies in 1996 to 11.9 in 2016 in the United States, and from 0.5 per 1000 pregnancies in 1996 to 9.8 in 2016 in Canada.

In the United States, rates of MRI use exceeded CT rates beginning in 2013. In Canada, this transition started in 2007.

Use of conventional x-rays doubled in the United States from 1996 to 1999, increasing from 34.5 per 1000 pregnancies to 72.6. It then decreased to 47.6 in 2016. These rates slowly increased in Ontario, from 36.2 per 1000 pregnancies in 1996 to 44.7 in 2016.

In most years, rates of use of angiography, fluoroscopy, and nuclear medicine were higher in the Ontario than in the United States. But overall rates remained low, at 5.2 per 1000 pregnancies.

Rates of imaging in the United States were higher for black, Native American, and Hispanic women than for white women. For example, 1.49% of black women underwent CT imaging, compared with 0.74% of white women.

Imaging rates were also higher among women with preterm birth and women who were younger than 20 years or older than 40 years.

The reasons for the racial and age differences in exposures to ionizing radiation during pregnancy are unclear, the authors state.

"Further investigation of variation in imaging use across patient subpopulations is needed to ensure that radiation is used only when necessary in pregnant women," they write.

The authors note several potential limitations. The study included only women who gave birth at 24 weeks' gestation or later, and conclusions cannot be drawn regarding pregnancies that did not end in live birth.

The study was supported by the National Cancer Institute and the Ontario Ministry of Health. One coauthor reports receiving personal fees from Bayer Healthcare. The other authors have disclosed no relevant financial relationships.

JAMA Netw Open. Published online July 24, 2019. Full text

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