Which of the following is approximately the least radiation absorbed dose to the fetus that could cause spontaneous abortion?

• A. 10 rads
• B. 25 rads
• C. 50 rads
• D. 100 rads

Answer: (C)

Deterministic effects from fetal irradiation have a threshold: the effect won’t occur until the dose reaches a certain level, and once you pass that threshold the likelihood increases with dose. For spontaneous abortion, that threshold is in the tens-of-rads range. Below that level, abortion is unlikely; at about that threshold, the risk becomes real, and higher doses increase the chance and severity of effects. Gestational age can influence sensitivity, but the key idea is that there’s a finite threshold for this outcome, and the dose around that threshold is the smallest dose capable of causing it.