Context: Women of Asian and South Asian descent are at increased risk of developing gestational diabetes mellitus (GDM) compared with Caucasians, despite lower body mass index (BMI). Nevertheless, there has been limited study of insulin action during pregnancy in these ethnic groups.

Objective: To compare insulin sensitivity in pregnancy in Asian, South Asian and Caucasian subjects and determine if the impact of obesity on insulin action is modified by ethnicity.

Design and Participants: A cross-sectional study was performed in outpatients undergoing oral glucose tolerance testing in late pregnancy. Participants were stratified into 3 groups: (i) Caucasian (n = 116); (ii) South Asian (n = 31); and (iii) Asian (n = 28).

Main Outcome Measure: Insulin sensitivity was measured using the IS_OGTT index of Matsuda and DeFronzo, previously validated in pregnancy.

Results: There were no significant ethnic differences in insulin sensitivity despite variation in pre-pregnancy BMI (Caucasians: 25.2 kg/m²; South Asians: 23.3; Asians: 21.4, overall P = 0.0001). On multiple linear regression analysis, the strongest independent determinants of IS_OGTT were GDM (t5.71, P < 0.0001) and BMI (t5.43, P < 0.0001). Importantly, both Asian (t2.87, P = 0.0047) and South Asian (t2.46, P = 0.015) ethnicity also emerged as negative, independent determinants of IS_OGTT. Furthermore, Asian ethnicity significantly modified the association of pre-pregnancy BMI with IS_OGTT (interaction term: t2.29, P = 0.0231)

Conclusions: Asian and South Asian ethnicity are both independently associated with increased insulin resistance in late pregnancy. Pre-pregnancy BMI has a much greater effect on insulin resistance in pregnancy in Asian women than in Caucasians. Ethnicity thus emerges as a factor that modulates the effect of obesity on insulin resistance in pregnancy.