RecruitingAt-the-Breast vs. Expressed Human Milk: Genesis of Infant Nutrition (BEGIN)The purpose of this study is to define human milk (HM) as an ecosystem which investigators will then combine into temporal models of milk dynamics to accurately describe HM chronobiology. This study addresses 4 crucial public health gaps: 1) how breast milk changes over time and over the day, 2) how milk dynamics are related to infant sleep patterns, 3) how milk dynamics are related to infant microbiome dynamics, and 4) how all these relationships differ between infants fed directly at-the-breast vs pumped milk. These fundamental insights have been unknown until now, so that families who feed pumped breast milk are completely underserved. These results are critical to optimizing infant feeding and health outcomes for all infants receiving breast milk.