Final answer:
Radiation exposure in the first trimester can lead to severe developmental defects or miscarriage because embryos have rapidly dividing cells and undergo critical development during this period.
Step-by-step explanation:
The most likely radiation-induced effect on a fetus during the first trimester is severe developmental defects or miscarriage. This is due to the rapid cell division and critical development occurring during this stage. Radiation can damage DNA and cause mutations in embryonic germ cells, with mutations at this early stage leading to widespread issues across various tissues and organs. Exposure to radiation during pregnancy, particularly in the first trimester, may also result in a range of effects from no observable effect to temporary or permanent sterility depending on the dose, and in high enough doses, can be fatal to the developing fetus.