BACKGROUND: Differences between mothers who do and do not succeed in breastfeeding are likely to confound associations of lactation with later maternal adiposity.

OBJECTIVE: We compared adiposity and blood pressure (BP) in women randomly assigned to an intervention to promote prolonged and exclusive breastfeeding or usual care.

DESIGN: We performed a cluster-randomized trial at 31 hospitals in Belarus in 1996-1997.

RESULTS: Of 17,046 women enrolled at delivery, we assessed 11,867 women (69.6%) at 11.5 y postpartum. The prevalence of exclusive breastfeeding ≥3 mo was 44.5% in 6321 women in the intervention group and 7.1% in 5546 women in the control group. At 11.5 y postpartum, mean (±SD) body mass index (BMI; in kg/m(2)) was 26.5 ± 5.5, the percentage of body fat was 33.6% ± 8.3%, and systolic BP was 124.6 ± 14.6 mm Hg. On intention-to-treat analysis (without imputation) with adjustment for clustering by hospital, mean outcomes were lower in intervention compared with control mothers for BMI (mean difference: -0.27; 95% CI: -0.91, 0.37), body fat (-0.49%; 95% CI: -1.25%, 0.27%), and systolic BP (-0.81 mm Hg; 95% CI: -3.33, 1.71 mm Hg), but effect sizes were small, CIs were wide, and results were attenuated further toward the null after adjustment for baseline characteristics. Results were similar in sensitivity analyses [ie, by using conventional observational analyses disregarding treatment assignment, instrumental variable analyses to estimate the causal effect of breastfeeding, and multiple imputation to account for missing outcome measures (n = 17,046)].

CONCLUSION: In women who initiated breastfeeding, an intervention to promote longer breastfeeding duration did not result in an important lowering of adiposity or BP. This trial was registered at clinicaltrials.gov as NCT01561612 and at Current Controlled Trials as ISRCTN37687716.