National breastfeeding surveillance questions in the U.S. ask solely about infant human milk (HM)-consumption, and not about mode of HM-feeding-at the breast or from a bottle. This is problematic because most HM-feeding mothers express their milk, as well as or instead of feeding at the breast, and expressed HM is fed to their infants from bottles. Thus, current breastfeeding surveillance questions do not fully describe the range of HM-feeding behaviors employed by families in the U.S. Our aims were to (1) explore practices for feeding HM at the breast and from a bottle qualitatively, and (2) develop and administer surveillance questions that elicit information about mode of HM-feeding, and use these questions to explore prevalence of HM expression and expressed HM-feeding among a national sample. Qualitative data came from semi-structured interviews with 41 mothers in upstate New York who had experience with HM expression. Interviews covered topics related to at-the-breast and expressed-HM feeding. We identified themes in the transcripts using inductive analysis. We used this formative qualitative data to develop a survey to explore at-the-breast feeding, HM expression, and expressed-HM feeding among a national sample of mothers. We administered the survey to a convenience sample of 451 mothers, recruited through ResearchMatch.org. Participants in our qualitative study described strategies for HM-feeding that ranged from predominant at-the-breast feeding to exclusive expressed HM-feeding. However, results from both studies indicated that the predominant strategy employed by HM-feeding mothers is a combination of both at-the-breast feeding and expressed-HM feeding. Among participants in our survey, the proportion of HM-fed infants consuming HM from a bottle was ~70% or greater at 3, 6, and 12 months. Our qualitative work highlighted that HM sharing is a complex behavior influenced by many contextual factors. Awareness of informal HM sharing was high in both our studies, and 7% of mothers in our quantitative sample had fed their infant another mother's HM. The complex range of practices for feeding HM described here highlight problems for studying, monitoring, and promoting optimal infant feeding behaviors. It is time for public health officials to recognize at-the-breast feeding and expressed-HM feeding as distinct behaviors, and count them separately. Both maternal HM expression and expressed-HM feeding require in-depth study so that the health outcomes associated with these behaviors can be understood and so mothers and families can be provided with evidence-based recommendations about optimal infant feeding practices. ; 2021-02-01