Breast-feeding and mental and motor development at 5 1/2 years

Objective: Breast-feeding is associated with better child development outcomes, but uncertainty remains primarily due to the close relationship between breast-feeding and socioeconomic status. This study assesses the issue in a low socioeconomic status sample where breast-feeding was close to universal. Methods: Seven hundred eighty-four Chilean children were followed longitudinally from infancy. All but four were initially breastfed, 40% nursed beyond 12 months, and infant growth was normal. Child development was assessed at 5 1/2 years by a cognitive, language, and motor test battery. The duration of breast-feeding as the sole milk source was analyzed as a continuous variable, adjusting for a comprehensive set of background factors. Results: The relationship between breast-feeding and most 5 1/2-year developmental outcomes was nonlinear, with poorer outcome for periods of breast-feeding as the sole milk Source for < 2 months or > 8 months-statistically significant for language, motor, and one comprehensive cognitive test, with a suggestive trend for IQ. Conclusions: The observed nonlinear relationships showed that breast-feeding as the sole milk source for < 2 months or > 8 months, compared with 2-8 months, was associated with poorer development in this sample. The latter finding requires replication in other samples where long breast-feeding is common and socioeconomic status is relatively homogeneous.