Sedentary behavior is associated with adverse health outcomes in the general population. Whether sedentary behavior during pregnancy is associated with newborn outcomes, such as birth size, is not established, and previous studies have been inconsistent. While previous research suggests that male and female fetuses respond differently to maternal behaviors, such as physical activity, the role of infant sex in sedentary behavior-birth size associations has not been examined. Participants in the Omega study, a cohort in Washington State (1996-2008), reported leisure time sedentary behavior (non-work time spent sitting), light intensity physical activity, and moderate/vigorous leisure time physical activity duration in the year before pregnancy (N = 1373) and in early pregnancy (N = 1535, mean 15 weeks). Offspring birth size was abstracted from delivery records. Non-parametric calibration weighting was used to assign adjustment weight (matching the distribution of sociodemographic and medical characteristics of the full cohort (N = 4128)) to participants with available sedentary behavior data. Weighted linear regression models were used to estimate mean differences in offspring birthweight, head circumference, and ponderal index (birthweight/length3) associated with leisure time sedentary behavior. Regression models were run overall and stratified by offspring sex. Isotemporal substitution modeling was used to determine mean differences in birthweight associated with replacing sedentary behavior with light or moderate/vigorous physical activity. On average, women spent 2.3 and 2.6 h/day in leisure time sedentary behavior during pre- and early pregnancy, respectively. There were no associations of pre-pregnancy leisure time sedentary behavior with mean birthweight, head circumference, or ponderal index (adjusted β = - 12, 95% CI: -28, 4.1; β = 0.0, 95% CI: -0.04, 0.1; and β = 0.1, 95% CI: -0.2, 0.4, respectively). Early pregnancy sedentary behavior was not associated with mean birth size. Associations of sedentary behavior with mean birth size did not differ by offspring sex. Replacing sedentary time with light or moderate/vigorous physical activity was not associated with mean birthweight. We did not observe associations of maternal sedentary behavior during pre- or early pregnancy with mean offspring birth size. Pre-pregnancy and early pregnancy sedentary behavior may have important adverse effects on maternal health, but our results do not support associations with mean offspring birth size.