Context Cesarean section (CS), breastfeeding, and geographic location can influence the infant microbiota. Objective In this systematic review, evidence of the association between mode of delivery and infant gut microbiota up to 6 months of age was evaluated, as was the role of breastfeeding in this association, according to PRISMA guidelines. Data Source The Pubmed, Web of Science, Scopus, Embase, Medical Database, and Open Grey databases were searched. Data Extraction A total of 31 observational studies with ≥2 infant stool collections up to the sixth month of age and a comparison of gut microbiota between CS and vaginal delivery (VD) were included. Data Analysis Infants born by CS had a lower abundance of Bifidobacterium and Bacteroides spp. at almost all points up to age 6 months. Populations of Lactobacillus, Bifidobacterium longum, Bifidobacterium catenulatum, and Escherichia coli were reduced in infants delivered by CS. Infants born by CS and exclusively breastfed had greater similarity with the microbiota of infants born by VD. Conclusions Species of Bifidobacterium and Bacteroides are potentially reduced in infants born by CS. Geographic location influenced bacterial colonization. Systematic Review Registration PROSPERO registration no. 42017071285

Objectives To evaluate the association of pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) on infant gut microbiota diversity and abundance and the role of breastfeeding on this association. Methods Prospective cohort conducted in Rio de Janeiro, Brazil. Maternal pre-pregnancy BMI (< 25; ≥ 25 kg/m², normal/excessive) and GWG adequacy (adequate; excessive) were the exposures and breastfeeding practice status (exclusively breastfeeding EBF; predominant/complementary feeding PCF) was analyzed as an effect modifier. Infant stool samples were collected for 56 infants between 26–45 days. Samples were sequenced using 16S rRNA gene sequencing (MiSeq). Analysis included alpha diversity indexes (Shannon, Faith-PD, and Observed species), beta diversity metrics and Wilcoxon-Mann-Whitney Test, linear regression, permutational multivariate analysis of variance and linear discriminant analysis effect size. Results A higher median alpha diversity in infants born from mothers with excessive GWG was observed (Mann-Whitney Test: P = 0.005) and infants born from mothers with excessive GWG were positively associated with alpha diversity (β = 0.351; SE = 0.146; p-value = 0.020). Gut microbiota of infants born from mothers with excessive pre-pregnancy BMI were enriched with Dialister genus and Lactobacillus Ruminis, Haemophilus Parainfluenzae and Veillonella Parvula species and those born from mother with excessive GWG had higher abundance of Staphylocococcus genus, Staphylococcaceae family, Bacillales order and Bacilli class. Infant gut microbiota diversity and abundance did not differ according to combined categories of pre-pregnancy BMI and breastfeeding status and GWG and breastfeeding. Conclusions Maternal gestational weight gain was associated with diversity of the infant gut microbiota. Breastfeeding did not an effect modifier in this association. Funding Sources Foundation for the Support of Research of the State of Rio de Janeiro, National Council for Scientific and Technological Development and Columbia University Grant.