BACKGROUND: Up to 25-35% of women receive antibiotics (ABX) during pregnancy, but little is known about the consequences on a key mucosal interface such as the mammary gland, and on the development of the neonatal gut's microbiota and IgA. We hypothesize that prenatal ABX negatively affect the immune functionality of mammary gland, the composition of breast milk microbiota, the development of neonatal fecal microbiota and the abundance of neonatal fecal IgA. METHODS: Case-control translational cohort study on women and neonates in the presence or absence (N = 41 + 41 pairs) of exposure to prenatal ABX for at least 7 consecutive days after 32 weeks of gestation. RESULTS: We will evaluate IgA concentration in breast milk and in neonatal feces up to one year after delivery. We will also evaluate clinical parameters, neurodevelopment and the composition of the IgA-coated and uncoated fractions of breast milk and fecal microbiota by means of magnetic-activated cell sorting (MACS) coupled with shotgun metagenomics. Finally, we will measure the concentration of the chemokine CCL28 on maternal serum and breast milk, as a marker of activity of the entero-mammary pathway. CONCLUSIONS: Our results might support a data-driven evaluation of breast milk immune function in women exposed to prenatal ABX. IMPACT: Breast milk IgA and microbiota are critical to determine the positive effects of breastfeeding in infants. This research protocol will investigate breast milk IgA, microbiota, and the IgA