Breast Milk Microbiota in Healthy Women: A Scoping Review of modulating factors and the dynamics of bacterial composition during lactation.

The human breast milk microbiota plays a key role in newborn microbial colonisation. Its composition varies throughout the postpartum period and is modulated by maternal and perinatal factors. This scoping review aimed to systematically map current knowledge of the breast milk bacterial microbiota in healthy lactating women and describe its compositional dynamics in relation to identified modulating factors. A systematic search was conducted across five databases (PubMed, ScienceDirect, Scopus, Google Scholar, and Web of Science) following the PRISMA-ScR guidelines targeting studies that used 16S rRNA sequencing. Twenty-three studies from America (6), Europe (5), Asia (11) and Oceania (1) published between 2019 and 2025 were selected, with sample sizes ranging from five to 334 breast milk samples per study. The most consistently reported genera were Streptococcus, Staphylococcus, Lactobacillus, Pseudomonas, and Acinetobacter. The most frequently reported and analysed associated factors across the 23 studies were lactation stage, maternal body mass index (BMI), and mode of delivery. However, the methodological heterogeneity across studies, particularly regarding the hypervariable region analysed, represents the main source of variability in the results. These findings enhance our understanding of breast milk microbiota dynamics and highlight how maternal and perinatal factors influence its composition.