Background: Sedentary behavior (SED) and moderate-to-vigorous intensity physical activity (MVPA) have important implications for health; however, little is known about predictors of these behaviors during pregnancy. Methods: This cohort study measured SED (activPAL) and MVPA (GT3X) in each trimester of pregnancy. Univariate associations of demographic, socioeconomic, and pregnancy health-related factors with SED or MVPA were calculated. Associations with P < .10 were included in stepwise linear regression models to determine independent predictors in each trimester. Results: Pregnant women (n = 127) were age 31.0 (4.9) years and 78% white. In regression models across trimesters, fewer children ≤ age 5 in the household (P < .04) and primarily sitting job activity (P < .008) were related to higher SED and use of assisted reproductive technology (P < .05) was associated with higher MVPA. In at least one trimester, younger age was related to higher SED (P = .014); no history of pregnancy loss (P < .04), being married (P = .003), employed (P < .004, full time or student), white race (P = .006), and higher education (P = .010) were associated with higher MVPA. Conclusions: Predictors of SED in pregnancy were more consistent, and differed from predictors of MVPA. These findings may help identify women at risk of high SED or low MVPA, though future research in larger samples is needed.