Pregnant women are at a high risk of falling. To our knowledge, ground reaction forces (GRFs) during stair locomotion have not been reported for pregnant women. PURPOSE: To examine the effects of advancing pregnancy on GRFs during stair locomotion. METHODS: Data were collected on 41 pregnant women in the mid 2nd and 3rd trimesters, and on 40 control women. Subjects walked at their freely chosen walking speeds during ascent and descent. A force plate imbedded in the second stair, but structurally independent of the staircase, was used to collect GRF data (1080 Hz). Three trials were collected from each subject. GRF variables were normalized to body mass. A two-factor ANOVA (trimester x subject) was performed on each of these vertical GRF variables: passive peak, time to passive peak, loading rate, min between peaks, time to min between peaks, active peak, time to active peak, and impulse. A second ANOVA was performed on each of these shear GRF variables: max braking force, time to max braking force, braking impulse, max propulsive force, time to max propulsive force, propulsive impulse, medial impulse and lateral impulse. (Bonferroni-corrected a = 0.006). Tukey post-hoc tests were performed when appropriate. RESULTS: Differences in GRFs between pregnant women in their 2nd and 3rd trimesters and control women were noted for both ascent and descent. CONCLUSIONS: Pregnant women demonstrated GRF alterations that may be related to a slower walking velocity or increased instability. In particular, the increase in the medial impulse during pregnancy may be related to 'waddling' and changes in frontal plane control.