Abstract Objective Physical activity is recommended for pregnant women without medical or obstetric complications. This study described the prevalence and correlates of objectively-measured physical activity and sedentary behavior among United States pregnant women. Methods Using cross-sectional data collected from the 2003 to 2006 National Health and Nutrition Examination Survey (NHANES), 359 pregnant women ≥16 years wore an accelerometer for 1 week. Results Women participated in a mean of 12.0 minutes/day (standard error (SE) 0.86) of moderate activity and 0.3 minutes/day (SE 0.08) of vigorous activity. Mean moderate to vigorous physical activity varied by trimester: 11.5 minutes/day in first trimester, 14.3 minutes/day in second trimester, and 7.6 minutes/day in third trimester. On average, women spent 57.1% of their monitored time in sedentary behaviors. In multivariable adjusted models, moderate to vigorous physical activity was higher in the first (p = 0.02) and second (p < 0.001) trimesters compared to the third trimester, and among women with higher household income (p = 0.03) compared to lower household income. In multivariable adjusted models, average counts/minute was higher in the second compared to the third trimester (p = 0.04). Conclusion Most pregnant women spent more than half of the monitored day in sedentary behaviors and did not meet recommendations for physical activity.