In Reply Velle-Forbord and colleagues raise concerns about reverse causation and differences in metabolic parameters among lactation groups. The longitudinal design in our study1 (up to 7 examinations in 30 years) addressed reverse causation by measuring antecedent risk factors, including insulin resistance and dysglycemia before pregnancy, as well as other potential confounders in the lactation-diabetes association. Although biochemical measures reached statistical significance, the absolute values of differences among lactation groups were quite small (<2% to 5%) and fell within normal physiologic ranges, as expected for young and healthy women enrolled in CARDIA at ages 18 to 30 years. Thus, lactation groups exhibited clinically similar metabolic status (eg, median homeostatic model assessment for insulin resistance range of 1.5-1.9; mean fasting glucose range of 79-80). Nevertheless, inclusion of baseline insulin resistance or fasting glucose in multivariate models only minimally affected the strong, graded inverse associations for lactation duration with incident diabetes.