Reactions to radiant heat stimuli were measured in pregnant women and in a control group of non-pregnant women matched in age. Receiver operating characteristic curve parameters were computed for each of the stages of pregnancy from the 6th month until the postpartum period. When painful stimuli were used, a group by stage of pregnancy interaction was found in the analysis on the response criterion for reporting pain. Pregnant women were found to be significantly more willing to label radiant heat stimuli as painful during the last 2 weeks of pregnancy than during any of the other stages of pregnancy. The control women tested during the same time sequence as the pregnant women showed stable response criteria across the 6 month testing period. There were no group or stage of pregnancy effects evident when the discrimination accuracy measures were analyzed; nor were any effects found with responses to thermal stimuli. The results suggest that the changes in pain reaction associated with pregnancy are based on non-sensory factors that influence the subject's willingness to report pain.