In this randomized, double-blind, placebo-controlled trial, 80 lactating mothers were randomly divided into two groups to receive daily supplement of synbiotic (n = 40) or a placebo (n = 40) for 30 days. Information on dietary intake was collected and anthropometric measurements were taken using standard calibrated instruments. Data analysis was carried out using nutritionist IV, SPSS and Epi Info software. Synbiotic supplementation resulted in a slight increase in mean energy intake while, in the placebo group, maternal energy intake decreased significantly (p < 0.023). Although maternal weight and BMI increased slightly in the supplemented group, these two parameters decreased significantly in the placebo group (p < 0.01). Also, infants' weight gain in the synbiotic group was significantly higher than the placebo group after the intervention (p < 0.044). Synbiotics may prevent weight loss in lactating mothers and result in weight gain in infants. Further experiments are required to study these effects in undernourished lactating mothers and their infants.