We evaluated the influence of maternal pre-pregnancy body mass index (BMI), based on reported pre-pregnancy weight and height, on blood pressure (BP) levels during pregnancy by using information from a prospective cohort of 1733 women recruited before 20 weeks' gestation. Maternal antenatal BP values were abstracted from medical records, and we evaluated the mean BP differences according to BMI group in regression models, using generalised estimating equations to account for repeated BP records within each pregnancy. In each trimester, mean systolic BP (SBP) and diastolic BP (DBP) values were positively associated with maternal pre-gestational BMI. This association persisted after adjustment for maternal age, parity, smoking, education, marital status and physical activity. Overweight women (25-29 kg/m(2)) had first-, second- and third-trimester mean SBPs that were 8.1, 7.7 and 8.2 mmHg, respectively, higher than values observed in lean women (<20 kg/m(2)). Mean DBP values were 4.5, 5.4 and 5.6 mmHg higher for each successive trimester in overweight vs. lean women. Obese (>30 kg/m(2)) women consistently had the highest mean SBP and DBP values. Trimester-specific mean SBP values were 10.7-12.0 mmHg higher among obese women vs. lean women. Corresponding trimester-specific mean DBP values were 6.9-7.4 mmHg higher in obese vs. lean women. Similar patterns were observed when trimester-specific average mean arterial pressures were evaluated. Elevated pregnancy BPs associated with maternal pre-gestational BMI are consistent with a large body of literature that documents increased pre-eclampsia risk among overweight and obese women.