Pregnant women are more than twice as likely to fall compared to non-pregnant women. The physiological and anatomical effects of pregnancy may alter the gait of pregnant women, increasing their risk of falling. To our knowledge, ground reaction forces (GRFs) during pregnancy have not been reported. PURPOSE: The purpose of this study was to examine the effects of pregnancy on GRFs during walking. METHODS: Data were collected on 41 pregnant subjects in the mid 2nd and 3rd trimesters, and on 40 control women. Subjects walked at their freely chosen walking speeds along a ten meter walkway in the laboratory. Two force plates, hidden in the walkway, were used to collect GRF data (1080 Hz). Three trials were collected from each subject. All GRF variables were normalized to body weight. An ANOVA was performed to determine if stance time differences existed between pregnant subjects in their 2nd trimesters, their 3rd trimesters, and the control group (a=0.05). A second ANOVA was performed on each of the following vertical GRF variables: passive peak, time to passive peak, loading rate, minimum between peaks, time to minimum between peaks, active peak, time to active peak, and impulse. Additionally, a third ANOVA was performed on each of the following shear GRF variables: maximum braking force, time to maximum braking force, braking impulse, maximum propulsive force, time to maximum propulsive force, propulsive impulse, medial impulse and lateral impulse. (Bonferroni-corrected a = 0.006). RESULTS: Pregnant women exhibited 6% longer stance times than non-pregnant women. They also demonstrated lower values for every GRF variable assessed, except for the minimum between the peaks, which was greater in the pregnant women. Each of the GRF peaks occurred later in the pregnant women. Total vertical GRF impulse was greater in the pregnant women. Only the medial impulse and the time to the passive peak were greater in the 3rd trimester when compared to the 2nd trimester. CONCLUSIONS: Pregnant women exhibited alterations in GRFs. Few differences were noted between trimesters. The GRF alterations may be due to the pregnancy, or may simply be due to a slower walking speed. Also, it is unknown if these changes predispose a woman to a fall or if they help to prevent falls. Further research is necessary to answer these questions.