To evaluate the relationship between prenatal metabolic markers and breastfeeding outcomes in women with gestational diabetes mellitus (GDM). Secondary analysis of a cluster-randomized trial of a lifestyle intervention to improve metabolic health among women with GDM. Women were enrolled between 22 and 36 weeks' gestation and followed through 10 months postpartum. Metabolic markers were measured at enrollment. Women reported when they stopped breastfeeding, whether they breastfed as long as desired, and when they introduced formula. We evaluated the association of tertiles of metabolic markers with undesired weaning and time to breastfeeding cessation using Cox proportional hazards models and Mantel-Haenszel chi-square tests, respectively. Eighty-two women were eligible for analysis. There was a statistically significant difference in time to breastfeeding cessation among tertiles of fasting glucose, hemoglobin A1c (A1c), body mass index (BMI), and skinfolds (all p < 0.05). Women with higher fasting glucose, BMI, or skinfolds were also more likely to report undesired weaning; women with higher fasting glucose introduced formula earlier. Higher fasting glucose, A1c, BMI, and subscapular skinfolds were associated with earlier breastfeeding cessation in women with GDM. These markers may identify mothers in need of enhanced postpartum support to achieve their breastfeeding goals.