Answer to Question #11738 Submitted to "Ask the Experts"

Category: Pregnancy and Radiation — Exposures to embryo/fetus

The following question was answered by an expert in the appropriate field:

Q

Last week I broke my back and was given a computed tomography (CT) scan of my lower abdomen, resulting in a dose of 11 milligrays (mGy). I told my doctors that I could be pregnant, and they did a serum pregnancy test. The result was negative so they proceeded with the CT scan, placing an x-ray shield over my uterus.

This week I found out I am pregnant but very early in my pregnancy, about four weeks. My thought is that when I was given the blood test, it was negative because it was too early—about two to three weeks into my pregnancy and prior to implantation.

I am very fearful about the exposure to the fetus. I am also confused about the conversion of milligray to millisievert. Any help would be greatly appreciated.

A

Thank you for your questions. The fetus will not be harmed by radiation from the CT scan.

You have several things going for you here. The first is that early in pregnancy (prior to implantation), there is an "all or none" response to radiation exposure. This means that if the radiation dose is high enough, there will be a spontaneous abortion. However, if that does not occur, the person is still pregnant and there are no effects from the radiation—now or in the future. The radiation dose of 11 mGy is about one-tenth the dose that would cause a spontaneous abortion in early pregnancy and so too low to cause that effect. Because you are still pregnant, we know this did not occur.

Also, even if the fetus was older and past the point where the "all or none" effect occurs, the radiation dose is too low to cause other harmful effects. It would take a significantly higher dose (50–100 times more) to cause harmful effects.

And there's the shield they gave you. If it's like the shields we use here, the radiation dose that you received under that shield is only about 5% of the radiation dose you received elsewhere (the 11 mGy). So if the fetus received any radiation dose at all, it would have been closer to 0.5 mGy. Although I doubt the fetus received even that because of abdominal tissue shielding it.

As for comparing milligray and millisievert: do you really want to know? Just kidding! I won't go too far into it, but the unit of milligray is a measure of the absorbed radiation dose or how much radiation energy was deposited in the tissue. The other unit, millisievert, is a unit that takes into account the absorbed radiation dose and the biological effectiveness of the type of radiation (how effective it is at causing damage). For our purposes, because this was x-ray radiation exposure, either term would be okay to use.

Kelly Classic
Certified Medical Health Physicist

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 12 September 2016. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.