Background. Available evidence suggests that prolonged and exclusive breastfeeding is associated with lower infant weight and length by 6 to 12 months of age. This evidence, however, is based on observational studies, which are unable to separate the effects of feeding mode per se from selection bias, reverse causality, and the confounding effects of maternal attitudinal factors. Design/Methods. A cluster-randomized trial in the Republic of Belarus of a breastfeeding promotion intervention modeled on the World Health Organization (WHO)/UNICEF Baby-Friendly Hospital Initiative versus control (then current) infant feeding practices. Healthy, full-term, singleton breastfed infants (n = 17 046) weighing greater than or equal to 2500 g were enrolled soon after birth and followed up at 1, 2, 3, 6, 9, and 12 months old for measurements of weight, length, and head circumference. Data were analyzed according to intention-to-treat, while accounting for within-cluster correlation. To assess the potential for bias in observational studies of breastfeeding, we also analyzed our data as if we had conducted an observational study by ignoring treatment, combining the 2 randomized groups, and comparing 1378 infants weaned in the first month and those breastfed for the full 12 months of follow-up with either greater than or equal to 3 months (n = 1271) or greater than or equal to 6 months (n = 251) of exclusive breastfeeding. Results. Infants from the experimental sites were significantly more likely to be breastfed (to any degree) at 3, 6, 9, and 12 months and were far more likely to be exclusively breastfed at 3 months (43.3% vs 6.4%). Mean birth weight was nearly identical in the 2 groups (3448 g, experimental; 3446 g, control). Mean weight was significantly higher in the experimental group by 1 month of age (4341 vs 4280 g). The difference increased through 3 months (6153 g vs 6047 g), declined slowly thereafter, and disappeared by 12 months (10564 g vs 10571 g). Analysis by z scores confirmed that infants in both groups gained more weight than the WHO/Centers for Disease Control and Prevention reference, with no evidence of undernutrition in the control group. Length followed a similar pattern. In the observational analyses, infants weaned in the first month were slightly lighter and shorter at birth and their weight-for-age and length-for-age z scores declined by 1 month, but they caught up to both experimental and the other observational groups by 6 months and were heavier and longer by 12 months. Among infants in the 2 prolonged and exclusive breastfeeding groups, weight-for-age z scores fell slightly between 3 and 12 months; length-for-age fell below the reference by 6 months with catch-up to the reference by 12 months. Head circumference showed no significant differences at any age between the 2 trial groups or among the observational groups. Conclusions. Our data, the first in humans based on a randomized experiment, suggest that prolonged and exclusive breastfeeding may actually accelerate weight and length gain in the first few months, with no detectable deficit by 12 months old. These results add support to current WHO and UNICEF feeding recommendations. Our observational analysis showing faster weight and length gains with early weaning and slower gains with prolonged and exclusive breastfeeding may reflect unmeasured confounding differences or a true biological effect of formula feeding.
