Radiation Risk of Medical Imaging During Pregnancy

Authors: Mr Anthony Wallace*
                            Ms Anne Hayton *

Which radiology tests are absolutely contraindicated during pregnancy?

No radiological examination is absolutely contraindicated during pregnancy. As there are no dose limits in the medical use of ionising radiation, the risk of the procedure must be weighed against the risk to the patient and her foetus if she does not have the procedure.

Which radiology tests (and under what circumstances) are relatively contraindicated during pregnancy?

Any radiological examination of the mother that does not involve the direct irradiation of the foetus will deliver a comparatively low dose to the foetus.
However, several tests need to be considered carefully, in particular those where:
the foetus is likely to be directly exposed to the radiation beam; or
the imaging modality involves nuclear medicine.

What are the radiology referral guidelines for women of reproductive age?

Table 2 shows the typical foetal doses of radiological procedures.
Potential foetal radiation dose needs to be considered when a test or procedure involves the uterus being in the primary radiation beam:
’For most diagnostic radiation exposures of women in the first 3–4 weeks post-conception when the pregnancy is unrecognised, the risks of childhood cancer will be very small (and probably much smaller than if the exposure had occurred later in pregnancy). However, those few examinations yielding foetal doses in excess of about 10 mGy (the highest dose group in the table) could involve levels of risk that should be avoided, if possible, even in unrecognised pregnancies.’
The radiation dose to the uterus, and thus to the foetus, dictates how carefully the possibility of pregnancy needs to be excluded before the patient has the test. If the uterus is exposed to ionising radiation or the patient is having any kind of nuclear medicine test/procedure, then exclusion of pregnancy is important. It is most important for those tests where uterine dose is highest (>10 mGy)2 (Group 3 in Table 2)
Ref: HPA: Protection of Pregnant Patients during Diagnostic Medical Exposures to Ionising Radiation (RCE-9) March 2009.

Table 2: Typical foetal doses and risks of childhood cancer for common radiology tests and procedures*
Grouped according to foetal dose, from lowest to highest

Table 2 provides an indication of the expected dose of radiation to a foetus or embryo associated with various examination types. The foetal dose estimates provided apply only to the early stages of pregnancy when the foetus is small. It can be seen that even for the highest dose examinations at the bottom of the table, the expected dose to the foetus or embryo for a single examination is well below the 100 mGy threshold.

2. Long-term risk
This is also known as stochastic risk. As explained above, currently it is only possible to estimate potential risks. These are the theoretical risks of cancer some time after birth (as a child or adult) and the risks of hereditary diseases occurring in the descendants of someone who was exposed to radiation as a foetus.

The scientific information available about the effect of small amounts of radiation on the fertilised egg before it implants in the uterus (in other words, within hours to days of conception) and in the first 3–4 weeks after conception is limited. For most diagnostic radiation exposures of women in the first 3–4 weeks after conception when the pregnancy is unrecognised, the risks of childhood cancer will be very small (and probably much smaller than if the exposure had occurred later in pregnancy).

Approximately 1 in every 500 children develops cancer during childhood (even in the absence of any exposure to radiation as a foetus). For the examinations in the lowest dose range in Table 2, the estimated potential additional risk (over and above the normal lifetime risk of developing a cancer) for the foetus is approximately 1 in 1,000,000 and for the highest dose examinations at the bottom of the table, the additional risk (over and above the normal lifetime risk of developing a cancer) is thought to be approximately 1 in 1000 to 1 in 200. This means that for a foetus exposed to one of the highest doses of radiation after the first 4 weeks, the risk of childhood cancer might be increased to 3 in 500.

This is still considered to be a very low risk. If a higher dose examination is needed, because the risk of not doing the test is very significant for the mother, then this is appropriate. If a woman has one of the higher dose tests before she knows she is pregnant, the increased risk to the foetus is still considered to be small. This would not be a reason on its own to consider pregnancy termination, because of fears about cancer risks for the baby or child.

Table 2: Typical foetal doses and risks of childhood cancer for common radiology tests and procedures*
Grouped according to foetal dose, from lowest to highest

*Advice from the (UK): Health Protection Agency, the Royal College of Radiologists and the College of Radiographers

Note that patients who are cognitively impaired, non-English speaking or adolescents might require extra diligence on the part of the referrer and the radiology practice to exclude pregnancy before proceeding with tests involving ionising radiation.
In reference to the attached decision flowchart, the list of tests and procedures provided is not complete. The patient should check with the practice or hospital about possible radiation exposure to her uterus from the test she is having if she thinks she could be pregnant.

What are the adverse effects of imaging tests or interventions if a patient is thinking about becoming pregnant? (Include effect of radio-iodine on the father)

Preconception radiation exposure of either potential parent’s gonads has not been shown to result in increased cancer or malformation of children.
Nevertheless, recommendations for females not to become pregnant for at least 6 months after radiotherapy with radioiodine are directed towards ensuring that the clinical condition is controlled, and that another treatment with radioiodine is not going to be required during a subsequent pregnancy.
For males intending to father a child, it is advised that they wait at least 4 months after receiving therapy with a radionuclide. This time period is greater than the life of a sperm cell.

What are the alternative imaging tests or interventions or that can be safely undertaken during pregnancy?

The principal alternative imaging modalities are MRI and/or ultrasound. Neither have any adverse effects on pregnancies.

Useful websites for more information about imaging tests or interventions involving radiation in pregnancy:

Diagnostic Imaging Pathways
International Atomic Energy Agency (IAEA)
Health Physics Society
Duke University
Perinatology
Centre for Disease Control
Health Protection Agency (HPA)

References:

Wagner L, Lester R, Saldana L. Exposure of the pregnant patient to diagnostic radiations, 2nd ed. Medical Physics Publishing, Madison, 1997.
Wallace A, Cain T. Radiation Risk of Medical Imaging for Adults and Children. Royal Australian & New Zealand College of Radiology, Sydney, 2009
ARPANSA. RPS 14.2 Safety Guide for Radiation Protection in Nuclear Medicine. Australian Radiation Protection & Nuclear Safety Agency, Yallambie, 2008; Contract No.: 2.
Release of patients after therapy with unsealed radionuclides. Ann ICRP 2004; 34:v–vi, 1–79.
Vock P (ed). X-Ray Imaging & Pregnancy: Justification and Optimisation of Exposure. European Society of Radiology, Vienna, 2010.
Health Protection Agency (HPA), “Protection of pregnant patients during diagnostic medical exposures to ionising radiation: advice from the health protection agency,” Documents of the Health Protection Agency: Radiation, Chemical and Environmental Hazards RCE 9, The Royal College of Radiologists and the College of Radiographers, 2009.

*The author has no conflict of interest with this topic.

Page last modified on 22/8/2017.