Abstract : The newborn's bacterial primo-colonization is influenced by the delivery mode and the mother's gut microbiota. Our study aimed to evaluate the composition of gut microbiota in a cohort of preterm newborns. A prospective cohort study was conducted at the National Reference Center for Neonatology and Nutrition, CHU IBN SINA Children's Hospital in Rabat. Stool samples were collected from the newborns at the time of their first bowel movement upon admission, and then twice a week for a duration of 3 weeks. The gut microbiota was identified using RT-PCR. During the first 48 hours of life, the predominant bacterial group was Proteobacteria. By the end of the first week, Enterobacteriaceae, including Escherichia Coli, had colonized the intestines of all newborns. In the second week, a greater diversity of bacteria colonized the intestines, including: Enterococcus spp., Enterobacteriaceae, and Escherichia Coli. As the third week progressed, additional bacterial species began to establish. The least frequently observed bacteria during the first three weeks of life were Bacteroides fragilis, Bifidobacterium spp., and Lactobacillus spp. A comparison of the gut microbiota based on delivery mode revealed that newborns delivered by cesarean section had a less diverse microbiota compared to those born vaginally. Molecular biology enhanced the microbiota study and the knowledge about its beneficial and damaging effects on human health. It was evident from this study that the progression of microbiota within the newborn's intestines takes time and is influenced by the mode of delivery, whether it is a natural birth or a cesarean section.