Background: As obesity rates continue to rise, it is increasingly important to understand factors that can influence body weight and growth, especially from an early age. The infant gut microbiota has broad effects on a variety of bodily processes, but its relation to infant growth is not yet fully characterized. Since the infant gut microbiota is closely related to breastfeeding practices and maternal health, understanding the relationship between these factors and infant growth may provide insight into the origins of childhood obesity. Objectives: Identify the relationship between human milk exposure, maternal pre-pregnancy body mass index (BMI), the infant gut microbiota, and 12-month-old BMI-for-age z-scores (12M BAZ) to identify key factors that shape infant growth. Methods: Two Michigan cohorts (ARCHGUT and BABYGUT) comprised of a total of 33 mother-infant dyads provided infant fecal samples at 12M. After DNA extraction, amplification, and sequencing of the V4 16S rRNA region using Illumina MiSeq v2 Chemistry, gut bacterial diversity metrics were analyzed in relation to human milk exposure, maternal pre-pregnancy BMI, and infant growth parameters. Results: Recent human milk exposure was inversely related to maternal pre-pregnancy BMI and most strongly associated with infant gut bacterial community membership and individual gut microbiota richness differences. Maternal pre-pregnancy BMI was not associated with the infant gut microbiota after adjusting for human milk exposure. However, maternal pre-pregnancy BMI was the only factor significantly associated with 12M BAZ. Conclusions: Human milk exposure is one of the central influences on the infant gut microbiota at 12M of age. However, the lack of association between the infant gut microbiota and 12M-old infant BAZ suggests that genetic, physiological, dietary, and other environmental factors may play a more direct role than the gut microbiota in determining infant BAZ at 12M.