Estimation of fetal weight (EFW) in women with prepregnancy diabetes is an important element of care. It is less accurate women with type 1 diabetes than in women without diabetes. Type 2 diabetes is increasingly common among women of reproductive age and accurate knowledge of fetal weight enables better care of the woman and her baby. We aimed to demonstrate differences in accuracy in ultrasound determined EFW between women with type 1 diabetes and type 2 diabetes. A retrospective cohort study was completed over a six year period comparing the accuracy of EFW and birthweight in women with type 1 diabetes to type 2 diabetes, adjusted for maternal body mass index, birthweight centile and time from scan until delivery. Of the 108 pregnancies included, 58 (53.7%) of the infants were born to women with type 1 diabetes and 50 (46.3%) to women with type 2 diabetes. Ultrasound EFW was more accurate in women with type 2 diabetes compared to women with type 1 diabetes when compared using absolute difference (224 grams, p<0.001, 95% CI 104-343) or percentage difference (5.8%, p<0.001, 95% CI 2.3-9.3) after adjustment for maternal and clinical factors. This was also reflected in the accuracy of ultrasound in predicting birthweight >4000gm, with a sensitivity of 57.1% in women with type 2 diabetes as compared to 13.3% in women with type 1 diabetes. Clinicians may be able to use this information to make clinical decisions incorporating EFW in women with type 2 diabetes with more confidence.