Background
Knowledge about expected insulin requirements during pregnancy, in women with pre‐existing diabetes may assist clinicians to effectively respond to gestation‐specific changes in glycemic pattern. Few studies have examined differences between type 1 (T1DM) and type 2 diabetes (T2DM).

Aims
To compare patterns of insulin requirements in pregnancy for women with pre‐existing T1DM and T2DM.

Material and Methods
A retrospective cohort study of 222 pregnancies was conducted in women with pre‐existing diabetes, (67 with T1DM, 155 with T2DM). Total daily insulin dose (TID) at the end of each trimester, recorded as units and units per kilogram (median, 25th–75th percentile) as well as percentage increase in insulin dose per trimester were compared.

Results
Women with T1DM had higher insulin requirements in the first two trimesters than those with T2DM (0.69 (0.58–0.85) vs 0.36 (0.0–0.7) units/kg in first trimester; 0.80 (0.62–0.95) vs 0.61 (0.27–0.95) units/kg, P < 0.005) in second trimester), but requirements in late pregnancy were similar (0.97 (0.69–1.29) vs 0.95 (0.53–1.32) units/kg, P = 0.54). Women with T2DM needed much greater increases in insulin per trimester compared to T1DM (P < 0.001). Women with T1DM had a net fall in insulin requirements (3.7% in the first trimester and 4.1% in the late third trimester) while those with T2DM did not. Conclusions This is the largest comparison study of insulin requirements in women with pre‐existing diabetes, highlighting important trimester‐specific differences between T1DM and T2DM to guide insulin titration during pregnancy. Our findings suggest a differential effect of pregnancy‐mediated insulin resistance by type of diabetes.