To develop an index to predict fetal overgrowth in pregnancies complicated by diabetes. Data were derived from a cohort of 275 women with singleton gestations in a collaborative diabetes in pregnancy program. Regression analysis incorporated clinical factors available in the first 20-30 weeks of pregnancy that were assigned beta-coefficient-based weights, the sum of which yielded a fetal overgrowth index (composite score). Fifty-one (18.5%) pregnancies were complicated by fetal overgrowth. The derived index included five clinical factors: age ≤ 30, history of macrosomia, excessive gestational weight gain, enlarged fetal abdominal circumference, and fasting hyperglycemia. Area under the curve (AUC) for the index is 0.88 [95% confidence interval (CI) 0.82-0.92]. Cut-points were selected to identify "high-risk" and "low-risk" ranges (≥ 8 and ≤ 3) that have positive and negative predictive values of 84% (95% CI 70-98%) and 95% (95% CI 92-98%), respectively. The majority of women in our cohort (n = 182, 66%) had a "low-risk" index while 9% (n = 25) had a "high-risk" index. Sub-analyses of nulliparous women and women with gestational and pre-gestational diabetes revealed that the overgrowth index was equally or more predictive when applied separately to each of these groups. This fetal overgrowth index that incorporates five clinical factors provides a means of predicting fetal overgrowth and thereby serves as a tool for targeting the allocation of healthcare resources and treatment individualization.