BACKGROUND Physical activity (PA) initiated in the second half of pregnancy does not affect (or modify) insulin sensitivity in normal weight women. However, this might be different in obese women with lower insulin sensitivity. OBJECTIVE To test the hypothesis that in overweight and obese women at risk for gestational diabetes, PA is related to improved maternal glucose and insulin metabolism in pregnancy. METHODS A longitudinal study, with measurements at 15, 24, and 32 weeks of gestation, was conducted. Time spent in moderate-to-vigorous PA (MVPA) was measured objectively. In regression models, the relationship between time spent in MVPA at week 15 and changes in MVPA from 15 weeks to 24 or 32 weeks with fasting glucose, glycosylated hemoglobin, fasting insulin, insulin sensitivity, and insulin response at week 24 or 32 was assessed. RESULTS In a group of 24 women, MVPA in early pregnancy was significantly associated with a reduced first- and second-phase insulin response at week 32. Women with larger decreases in MVPA throughout pregnancy had significantly higher fasting insulin levels, worse insulin sensitivity, increased first- and second-phase insulin response, and higher triglyceride levels in late pregnancy compared to women with smaller decreases in MVPA. Time spent in MVPA was not related to glucose or glycosylated hemoglobin levels. CONCLUSIONS In our group of overweight and obese pregnant women, MVPA was associated with improved insulin sensitivity, insulin response, and decreased triglycerides at 32 weeks of pregnancy. This supports efforts for counseling obese women at risk for gestational diabetes in pregnancy to maintain their MVPA levels.