By Rebecca H. Allen, MD, MPH

Associate Professor, Department of Obstetrics and Gynecology, Warren Alpert Medical School of Brown University, Women and Infants Hospital, Providence, RI

Dr. Allen reports she receives grant/research support from Bayer and is a consultant for Merck.

SYNOPSIS: This retrospective cohort study estimated that the use of CT scans has increased 3.7-fold in the United States and 2-fold in Ontario, Canada, from 1996 to 2016. Overall, 5.3% of pregnant women in the United States and 3.6% in Ontario underwent imaging with ionizing radiation.

SOURCE: Kwan ML, Miglioretti DL, Marlow EC, et al. Trends in medical imaging during pregnancy in the United States and Ontario, Canada, 1996-2016. JAMA Netw Open 2019;2:e197249.

This retrospective cohort study was performed at six U.S. healthcare systems (Kaiser Permanente of Northern California, Northwest, Washington, and Hawaii; Marshfield Clinic in Wisconsin; and Harvard Pilgrim in Massachusetts) and in Ontario, Canada, to evaluate radiology exposures in pregnant women between Jan. 1, 1996, and Dec. 31, 2016. Eligible women were required to be enrolled in the health system for their entire pregnancy and had to give birth to a neonate of at least 24 weeks’ gestational age. Dates and types of all imaging studies performed during the pregnancy were collected, including computed tomography (CT), magnetic resonance imaging (MRI), radiography, angiography, fluoroscopy, nuclear medicine, and ultrasound. Imaging procedures performed for radiation treatment for cancer or with biopsies were excluded.

A total of 3,497,603 pregnancies from 2,211,789 women were included, with 26% from U.S. sites. Overall, 5.3% of patients from U.S. sites and 3.6% in Ontario underwent imaging with ionizing radiation and 0.8% in U.S. sites and 0.4% in Ontario underwent CT. In U.S. sites, CT use rates increased from 2.0 studies per 1,000 pregnancies in 1996 to 9.3 studies per 1,000 pregnancies in 2016 (3.7-fold). The use of chest CT, in particular, increased in U.S. sites from 0.2 per 1,000 to 4.0 per 1,000 pregnancies. The rate of MRI use also increased in the United States from one study per 1,000 pregnancies in 1996 to 11.9 per 1,000 pregnancies in 2016. The use of MRI for the abdomen and pelvis increased the most, from 1.1 per 1,000 pregnancies to 5.5 per 1,000 pregnancies. Radiography, angiography, fluoroscopy, and nuclear medicine imaging rates remained stable over time.


The authors of this study used health system records to estimate the increase in the use of imaging studies among pregnant women over a 21-year period in the United States and Canada. There are some limitations, in that only pregnant women who had a live birth greater than 24 weeks’ gestation were included, and researchers did not have information on the indications for the studies. However, the data do provide reliable information regarding the trend in imaging use over time using multiple sites that are attributed likely generalizable. The authors attribute this increase to multiple potential causes, such as advances in imaging technology, patient and physician demand, and defensive medicine.

The use of X-rays and CT scans involves ionizing radiation, and the risk to the fetus depends on the gestational age and dose of radiation. The threshold dose for causing congenital anomalies during the period of organogenesis (4 to 10 weeks’ gestational age) is estimated at 200 mGy. The dose for causing intellectual disability at 10 to 17 weeks’ gestational age is estimated at 60 to 310 mGy.1 A chest X-ray with two views exposes the fetus to only 0.0005 to 0.01 mGy. A chest CT exposes the fetus to 0.01 to 0.66 mGy, and a pelvic CT approaches 2.5 to 50 mGy. The exposure from CT can be modulated by the number and spacing of the images. Given these numbers, the use of chest CT for the evaluation of pulmonary embolus in pregnant women is appropriate. According to this study, chest CT for this indication has become more popular in the United States than the traditional ventilation-perfusion scanning compared to Ontario, Canada. While iodinated intravenous contrast for CT scans has not been shown to be harmful to the fetus, it is recommended for use only if absolutely needed.2

As this study shows, the use of MRI, especially for the abdomen and pelvis, has increased in pregnant women over time and surpasses the use of CT. MRI has the advantage of not using ionizing radiation and is considered safe for the fetus. MRI can be used in the evaluation of acute appendicitis in pregnancy or for placenta accreta, especially if ultrasound is unable to provide the detail necessary. Unlike CT, MRI typically can visualize structures without the use of contrast. The use of gadolinium-based contrast, when needed for additional imaging, has been controversial in pregnancy. Gadolinium is water-soluble and can cross the placenta and enter the fetal circulation and amniotic fluid. It is administered in a chelated form because free gadolinium is toxic. The duration of fetal exposure to gadolinium is of concern because it may disassociate into a free form the longer it remains in the amniotic fluid. Nevertheless, the American College of Obstetricians and Gynecologists states that gadolinium can be used if it is expected to improve interpretation of the MRI significantly and will benefit maternal or fetal outcome.2

During the course of caring for pregnant women, diagnostic imaging studies sometimes are necessary to assist with the evaluation of acute and chronic conditions. While the overuse of diagnostic imaging should be avoided, it is also important not to unnecessarily restrict access to studies that are needed, even if a woman is pregnant. Ultrasound and MRI avoid ionizing radiation and typically are not restricted in pregnancy. X-rays expose the fetus to minimal amounts of radiation. CT scans of the abdomen and pelvis are of the most potential concern depending on the settings used, but even these might be indicated for maternal health. Overall, providers should be judicious in their recommendations for diagnostic imaging tests and only use them if benefit to the patient will result.


  1. Tremblay E, Thérasse E, Thomassin-Naggara I, Trop I. Quality initiatives: Guidelines for use of medical imaging during pregnancy and lactation. Radiographics 2012;32:897-911.
  2. American College of Obstetricians and Gynecologists. Committee Opinion No. 723: Guidelines for diagnostic imaging during pregnancy and lactation. Obstet Gynecol 2017;130;e210-e216.