We studied the effects of presentation formats (frequency, 1-in-N, and visual) and numeracy level on students' understanding of prenatal screening results, as well as their risk assessment for having a fetus with Down syndrome. Frequency format (vs. 1-in-N and visual formats) improved participants' ability to accurately assess the chances of having a fetus with Down syndrome, and was associated with lower risk estimates. High numeracy levels were associated with a better ability to judge risk likelihood. For individuals of low numeracy levels, however, the frequency format significantly facilitated accurate understanding of probability information. This suggests that presenting information in frequency format is particularly beneficial for certain populations.