ABSTRACT Undernourished women in low-and-middle income countries (LMICs) experience greater fetal growth restriction in pregnancy and poorer birth outcomes, including preterm birth and small-for-gestational age (SGA). Infants born preterm and/or SGA are more vulnerable to stunting and inadequate neurocognitive development, with lasting effects through adulthood. Nutritional interventions targeted in pregnancy and lactation have the potential to break the intergenerational cycle of malnutrition. Recently, an expert consensus developed specifications for a fortified BEP “ready-to-use” food supplement for pregnant and lactating women in LMICs, which requires testing in undernourished settings for efficacy in improving birth and growth outcomes. Currently, there is no recommendation for supplementation for women who are breastfeeding, despite high rates of growth faltering in early life in LMICs, where maternal undernutrition is high and exclusive breastfeeding is recommended. Our research aims to 1) Evaluate the effect of daily BEP++ in pregnancy and/or lactation on infant growth outcomes in the first 6 months of life; 2) Evaluate the effect of daily BEP++ in pregnancy and/or lactation on maternal nutritional status and explore potential mediation pathways on infant growth outcomes; and 3) In a biospecimen sub-study, explore relationships between maternal and infant nutrient status, inflammation, human milk composition, and gut microbiome by supplementation in pregnancy and/or lactation vs. not. We propose a community-based, randomized controlled trial of daily BEP++ supplementation during lactation in rural Nepal (proposed addition). Our study population has high rates of maternal undernutrition, adverse birth outcomes, and poor infant growth, and is representative of rural settings in South Asia. To conduct this trial, we will leverage the infrastructure of our community-based field site and a randomized trial of a BEP++ supplement in pregnant women scheduled to begin in 2022 (parent trial). The parent trial will recruit pregnant women in pregnancy, randomize them to supplement in pregnancy or no supplement, and follow-up until birth to assess SGA incidence. We propose to 1) add a postpartum supplementation trial following supplementation in pregnancy, using a 2x2 factorial design, to evaluate the BEP++ supplement during lactation and measure infant growth outcomes at 6 months of life, 2) double the overall sample size of pregnant women enrolled to allow for detection of meaningful differences between postpartum supplementation, and 3) nest a biospecimen sub-study within the trial to explore aims on the causal relationships between maternal and infant nutritional status and infant growth. A sample of 2,000 pregnant women will yield 1,600 live births and 1,440 infants followed until 6 months (accounting for pregnancy loss and loss to follow-up). This will allow us to detect a difference for our primary outcome, mean length-for- age Z score at 6 mon...