The risk your unborn baby faces from radiation depends on his or her stage of development.

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How can radiation affect a fetus at its various gestational stages?

The time during which a fetus develops inside the mother's womb is vital for healthy growth. Radiation exposure before a baby's birth can increase the likelihood of disease -- such as cancers later in life -- or even end the child's life before it's barely begun. Since unborn babies are especially sensitive to radiation, it's best if an examination that involves radiation exposure can be postponed or replaced with another until after the baby is born [source: Classic].

Even so, the risk depends on the amount of radiation the unborn child is exposed to. The CDC measures the threshold for safe exposure by comparing it to the amount of radiation in 500 chest X-rays or less (.1 to .6 mSv x 500), which is deemed a low-dose. A high dose of radiation is considered to be the amount in 5,000 chest X-rays or higher [source: CDC]. Should an unborn baby be exposed to 500 chest X-rays worth of radiation or less at one time, the only health concern is that their risk of cancer could increase by less than 2 percent than the average rate.

  • The first couple weeks of pregnancy: Radiation exposure causes the greatest concern during the early weeks of pregnancy and can have an all-or-nothing effect on the embryo during the first eight days following conception [source: Health Physics Society]. During this time, the embryo is comprised of only a few cells, so a large dose of radiation dose could trigger a miscarriage or have no effect on the embryo at all.
  • Between 2 and 18 weeks of pregnancy: Around the eighth week of pregnancy, an embryo becomes a fetus. Around this time, there can be harmful health impacts to an unborn baby should it be exposed to radiation levels above the amount in 500 chest x-rays. The possibilities include brain damage, and a wide range of birth defects.
  • Between 18 weeks and full-term pregnancy: At this point in pregnancy, the fetus has outgrown its most fragile development stages. A mother-to-be can breathe a sigh of relief now that her unborn child is no longer any more susceptible to radiation than a newborn. Of course this doesn't mean all is free and clear, just a bit more protected.
  • 26 weeks and beyond: A fetus has now developed enough to resist many of the birth defects linked to pregnancy and radiation exposure since it is fully developed, although it will continue growing longer and heavier.

For more detailed information about how radiation levels can affect a fetus at different stages of development, see the CDC's"Radiation and Pregnancy" fact sheet.