A Health Care Personnel Guide For Assessing Radiation Risk And Selecting Imaging Procedures In Pregnant Women Easy-print edition of this Guide

This reference material is for use by medical staff affiliated with Brigham and Women's Hospital only. It is designed to inform your decision-making as you determine individual risks associated with medical imaging and communicate with your patients. It should not be interpreted as medical advice, nor is it necessarily germane to institutions using other imaging protocols. Patients with questions regarding these issues should speak directly with their caregivers. An additional resource is the Ask the Experts-Pregnancy and Radiation module of the Health Physics Society: http://www.hps.org
Fetal Risk Categories

To determine the radiation dose to the fetus, the cumulative radiation exposure from all the radiation exams the patient receives needs to be totaled. The fetal radiation recommendations of the National Council on Radiation Protection and Measurements (NCRP) were published in 19771. Essentially this report concludes that the risk of fetal abnormality is a function of the stage of development irradiated, and the total radiation dose received.

Radiation-related risks throughout pregnancy vary according to gestational age. For a given radiation dose, the risk to the fetus is most significant during the first trimester, somewhat less in the second trimester, and least in the third trimester. Malformation of organs (3rd - 8th weeks after conception) appears to have a threshold of 100 mGy. The risk for severe mental retardation (8th - 15th weeks) has a threshold of 100 mGy. These are considered to be acute radiation effects.

The fetal dose is considered negligible at less than 50 mGy when compared with the other risks of pregnancy. The risks of organ malformations during organogenesis (3rd to 8th week) is significantly increased above control levels only at doses above 150 mGy. Therefore, the radiation dose received from a diagnostic imaging procedure should not be the sole determinant when contemplating the termination of a pregnancy. Other risks, such as acute viral disease, teratogenic drug use, or when life of the mother is threatened, should also be considered in this regard.

The following table lists the fetal radiation risk categories according to fetal radiation dose and post-conception age:

Table 1: Fetal Risk Categories*
Gestational Age
< 100 mGy
≥100 mGy
< 2 weeks
I
III†
3 - 8 weeks
I
III‡
8 - 15 weeks
I
III§
> 15 weeks
I
II§

I = negligible risks
II = potential association with adverse birth outcome, especially with other teratogenic risk factors
III = highest risk condition.

* Adapted from United Nations Scientific Committee on the Effects of Atomic Radiation.2

† At 0-2 weeks post-conception, doses ≥100 mGy have an "all or none effect"
potentially causing embryologic demise, surviving fetus will progress to term
without associated effects

‡ At 3-8 weeks post-conception, doses ≥100 mGy have potential for organ malformation

§ 100 mGy threshold dose for mental effects