Lactating compared with nonlactating women display more favorable metabolic parameters, including less atherogenic blood lipids, lower fasting and postprandial blood glucose as well as insulin, and greater insulin sensitivity in the first 4 months postpartum. However, direct evidence demonstrating that these metabolic changes persist from delivery to postweaning is much less available. Studies have reported that longer lactation duration may reduce long-term risk of cardiometabolic disease, including type 2 diabetes, but findings from most studies are limited by self-report of disease outcomes, absence of longitudinal biochemical data, or no assessment of maternal lifestyle behaviors. Studies of women with a history gestational diabetes mellitus (GDM) also reported associations between lactation duration and lower the incidence of type 2 diabetes and the metabolic syndrome. The mechanisms are not understood, but hormonal regulation of pancreatic β-cell proliferation and function or other metabolic pathways may mediate the lactation association with cardiometabolic disease in women.