To describe breastfeeding rates from early to late infancy and to examine associations between breastfeeding duration and infant growth, including rapid weight gain (RWG, > 0·67 SD increase in weight-for-age Z-score), among infants from low-income, racially and ethnically diverse backgrounds.

A short, prospective cohort study was conducted assessing breastfeeding status at infant ages 2, 4, 6, 9 and 12 months. Infant length and weight measurements were retrieved from electronic health records to calculate weight-for-length Z-scores and the rate of weight gain.

Pediatric clinic in the Southeastern USA.

Mother-infant dyads (n = 256).

Most participants were African American (48 %) or Latina (34 %). Eighty-one per cent were participating in the Special Supplemental Nutrition Program for Women, Infants and Children. Infants were breastfed for a median duration of 4·75 months, with partial more common than exclusive breastfeeding. At 12 months, 28 % of the participants were breastfeeding. Infants breastfed beyond 6 months had significantly lower growth trajectories than infants breastfed for 0–2 months (β = 0·045, se = 0·013, P = 0·001) or 3–6 months (β = 0·054, se = 0·016, P = 0·001). Thirty-six per cent of the infants experienced RWG. RWG was more common among infants who were breastfed for 2 months or less than 6+ month breastfed group (relative risk = 1·68, CI95 (1·03, 2·74), P = 0·03).

Breastfeeding beyond 6 months is associated with the prevention of accelerated growth among infants from low-income, racially and ethnically diverse backgrounds, suggesting progress toward health equity.