Microbial colonization of the neonate is an important feature of normal birth. The gut microbiota has a central role in the programming of the host’s metabolism and immune function, with both immediate and long-term health consequences. During vaginal birth, the infant is exposed to diverse maternal microbes, of which specific faecal microbes colonize the infant’s gut. C-section eliminates the infant’s contact with maternal microbes, preventing vertical transmission of gut microbes. Consequently, infants are colonized by bacteria from the environment, including potential pathogens from the hospital environment. Recent studies have shown that intrapartum antibiotic exposure has a C-section-like effect on the infant gut microbiota. While the composition of the gut microbiota largely normalizes during the first year of life, epidemiological studies suggest that the aberrant early microbial exposures have long-term immunological and metabolic consequences. Because of the high prevalence of procedures that prevent normal gut microbiota development, effective methods to normalize the gut microbiota of neonates are urgently needed. Even more importantly, attention should be paid to the microbiota imbalance in C-section-born and antibiotic-exposed infants in clinical practice. Breastfeeding and probiotics are particularly important for infants with disrupted gut colonization.