Objective To assess median and percentile birthweight distribution in women with various groups of heart disease relative to a contemporaneous comparison group. Methods Data on birth weight and gestational age were collected from 1321 pregnancies ≥24 weeks' gestation in 1053 women with heart disease from seven UK maternity units. Women were assigned to one of 16 groups according to their cardiac lesion. In units where it was possible, data on two births, one delivering before and one after index cases, were collected, giving 2307 comparators. Birthweight percentiles (corrected for gestational age, sex and parity) were calculated using Aberdeen norms. We assessed the association of birth weight with cardiac lesion, maternal hypoxaemia (saturations <90%), systemic ventricular function and beta-blockers. Results 1321 pregnancies in women with heart disease and 2307 comparators were studied. Almost all groups with heart disease had lower median and percentile birth weights than comparators, significantly in 10 groups, the biggest effect seen in women with Fontan circulation, pulmonary hypertension, prosthetic heart valves, systemic right ventricle, Marfan syndrome, repaired tetralogy of Fallot and cardiomyopathy (in that order). In 307 pregnancies, women took beta-blockers; median birth weight adjusted for maternal age, parity and the effect of the cardiac lesion was 3116.7 g (IQR 790.4) when beta-blockers were used and 3354.3 g (IQR 634.1) when they were not (p<0.001). 17 women had saturations <90%, and median birth weight was significantly lower, 3105.4 g (IQR 1288.9) versus 3387.7 g (IQR 729.8) (p=0.006). Conclusion Our findings identify specific groups of women with heart disease at risk of having a small baby.