Association of gestational diabetes with other lactation-related disorders: A 2-sample Mendelian randomization analysis of causal effects.

Gestational diabetes mellitus (GDM) is associated with adverse metabolic outcomes and may also be related to postpartum breast and lactation disorders. Observational studies are susceptible to confounding and reverse causation. We used a 2-sample Mendelian randomization design to examine the association between genetic liability to GDM and other disorders of the breast and lactation associated with childbirth. Genetic liability to GDM was associated with higher odds of a composite phenotype of breast and lactation disorders associated with childbirth. Replication using independent samples and more specific clinical outcomes is required. Summary statistics for GDM and the FinnGen outcome "other disorders of breast and lactation associated with childbirth" were obtained from the Integrative Epidemiology Unit open genome-wide association study resource. single nucleotide polymorphisms associated with GDM (P < 5 × 10-6) were clumped (R2 < 0.001) within 10,000 kb. The inverse-variance weighted method was the primary analysis; Mendelian randomization-Egger, weighted median, simple mode, and weighted mode analyses were complementary methods. Heterogeneity, directional horizontal pleiotropy, leave-one-out, and Steiger directionality analyses were performed. Nineteen single nucleotide polymorphisms were retained; F statistics ranged from 21.08 to 288.04. Genetic liability to GDM was associated with higher odds of the outcome in the inverse-variance weighted analysis (odds ratio [OR], 1.50; 95% confidence interval [CI], 1.08-2.09; P = .0165). The weighted median (OR, 1.76; 95% CI, 1.09-2.84; P = .0216) and weighted mode estimates (OR, 1.98; 95% CI, 1.21-3.26; P = .0148) were directionally consistent. There was no statistical evidence of heterogeneity or directional horizontal pleiotropy. The Steiger test supported the direction from GDM to the outcome (P = 1.30 × 10-11).