OBJECTIVE: To describe associations among background determinants of physical activity, modifiable theoretical determinants of physical activity, and measures of physical activity during the second and third trimesters of pregnancy among Black women. DESIGN: Longitudinal cohort. SETTING: Medical center obstetric clinic. PARTICIPANTS: Pregnant Black women (n = 40 in second trimester, n = 38 in third trimester) METHODS: We measured background determinants (demographic and pregnancy characteristics, discrimination, and neighborhood walkability) during the second trimester. We measured modifiable theoretical determinants (self-efficacy and social support) and physical activity using self-report and device measures during the second and third trimesters. We used paired t tests to determine differences in the modifiable theoretical determinants from the second trimester to third trimester and used Pearson correlations among background and modifiable determinants and physical activity measures during the second trimester. RESULTS: Participants' physical activity levels were low during the second and third trimesters (32% and 22% met recommendation, respectively). We found no changes in self-efficacy or social support between trimesters and found no associations between these modifiable determinants and actual physical activity. We found a positive correlation between previous pregnancies and physical activity measured by devices, r(36) = .33, p = .048. Pregnancy-specific stress, r(38) = -.40, p = .013, was negatively correlated, and age, r(38) = .38, p = .017, was positively correlated with self-reported physical activity. CONCLUSIONS: Low levels of physical activity during pregnancy coupled with the absence of an association with modifiable factors affecting pregnancy physical activity indicate a need to further examine the social, cultural, and environmental determinants of physical activity.