Numerous anatomical changes occur during pregnancy that may be related to gait alterations, which in turn may be related to increased reports of lumbar and pelvis pain during pregnancy. While other researchers have reported changes in hip and ankle biomechanics, none have quantified torso kinematics related to the "waddling gait" that pregnant women are anecdotally said to exhibit. PURPOSE: The purpose of this study was to examine the effects of pregnancy on torso mechanics during gait. METHODS: Data were collected on 29 pregnant subjects in the mid-second and third trimesters and on 40 control women. An 8 camera motion capture system (120 Hz) was used to collect data of subjects walking at their freely chosen speed along an 8 meter laboratory runway. Subjects wore a modified Helen Hays marker set. Right foot heel strike (RHS) and left foot toe off (LTO) were determined from force plate data (1080 Hz). The 3D angles of the thorax (i.e. upper torso) and pelvis were determined at RHS. The frontal plane movement of the C7 marker and the ranges of motion of the thorax and pelvis during gait were determined between RHS and LTO. An ANOVA was performed to determine if differences existed between pregnant women in their second trimester, third trimester, and controls (á=0.05). RESULTS: There was significantly (p<0.01) more frontal plane motion of C7 between the third trimester (6.5 +/- 2.7cm), second trimester (5.6 +/- 1.9 cm), and controls (4.7 +/- 1.8cm) during the stride. At RHS, the sagittal plane position of the thorax was more extended (p<0.01) as pregnancy advanced (third trimester: -6.6 +/- 4.5 degrees, second trimester:-3.5 +/- 5.1 degrees, Con: 1.7 +/- 5.1 degrees). No other differences in thorax and pelvis mechanics were noted. CONCLUSIONS: Pregnant women demonstrated more frontal plane motion of C7 during gait, particularly in the third trimester. Because no differences in frontal plane angles of the thorax and pelvis were seen between groups, this movement of C7 is likely due to a side-to-side shifting of the body rather than a leaning. Pregnant women demonstrated a backward leaning of the thorax, which is likely to counterbalance a forward position of the center of mass due to increasing abdominal size.