Objective Obesity is known to be associated with cardiovascular compromise and is a major risk factor for the development of hypertensive disorders in pregnancy. The aim of this study was to investigate the effect of obesity on the maternal cardiovascular system. Design This was a prospective, observational, longitudinal study. Setting A tertiary centre in London Population Pregnant women with booking body mass index (BMI) ≥ 30kg/m2 (n=64) were compared to pregnant women with normal booking BMI (20-24.9kg/m2) (n=14). Methods Two-dimensional trans-thoracic echocardiography. Main outcomes Longitudinal difference in blood pressure, cardiac geometry and cardiac function between the groups. Results In women with obesity, the blood pressure, heart rate and cardiac output were higher and peripheral vascular resistance was lower (p<0.01 for all) compared to normal BMI women. Women with obesity had altered cardiac geometry with higher left ventricular end diastolic diameter, relative wall thickness and left ventricular mass (p<0.001 for all comparisons). There was also evidence of impaired diastolic indices in the obese group with lower E/A ratio, TDI E’ lateral and medial and higher left atrial volume (p<0.01 for all). Finally, women with obesity had reduced longitudinal function between the second and third trimester of pregnancy indicating possible early cardiac dysfunction in this group. Conclusions Obesity is associated with maternal hyperdynamic circulation, altered cardiac geometry and suboptimal diastolic function compared to normal BMI pregnant women; this may contribute to the increased risk of complications in obese pregnant women. Funding UK charities: Borne and CW+