Breastfeeding involves signaling between mother and offspring through biological (breast milk) and behavioral pathways. This study tested this by examining the effects of a relaxation intervention in an understudied infant population. Breastfeeding mothers of late preterm (34^0/7-36^6/7 weeks) and early term (37^0/7-38^6/7 weeks) infants were randomized to the relaxation group (RG, n = 35), where they were asked to listen to a meditation recording while breastfeeding from 3 weeks post-delivery, or the control group (CG, n = 37) where no intervention was given. Primary outcomes-maternal stress and infant weight-were assessed at 2-3 (baseline) and 6-8 weeks post-delivery. Secondary outcomes included infant length, infant behavior, maternal verbal memory, salivary cortisol, and breast milk composition. Infants in the RG had significantly higher change in weight-for-age Z-score compared to those in CG (effect size: 0.4; 95% CI: 0.09, 0.71; p = 0.01), and shorter crying duration [RG: 5.0 min, 0.0-120.0 vs. CG: 30.0 min, 0.0-142.0; p = 0.03]. RG mothers had greater reduction in cortisol (effect size: -0.08 ug/dL, 95% CI -0.15, -0.01; p = 0.03) and better maternal verbal learning score (effect size: 1.1 words, 95% CI 0.04, 2.1; p = 0.04) than CG mothers, but did not differ in stress scores. A simple relaxation intervention during breastfeeding could be beneficial in promoting growth of late preterm and early term infants. Further investigation of other potential biological and behavioral mediators is warranted.