This study investigates the remodeling of the gut microbiome and metabolic changes during pregnancy. The researchers characterized the fecal bacteria of 91 pregnant women with varying pre-pregnancy body mass indices (BMIs) and gestational diabetes status, as well as their infants. They found that similarities between infant and maternal microbiotas increased with the age of the child, and the infant microbiota was not affected by the mother's health status. The gut microbiota underwent significant changes from the first (T1) to the third (T3) trimesters, with a dramatic expansion of diversity between mothers, an overall increase in Proteobacteria and Actinobacteria, and reduced richness. T3 stool showed stronger signs of inflammation and energy loss, but the microbiome gene repertoires remained constant between trimesters. When transferred to germ-free mice, T3 microbiota induced greater adiposity and insulin insensitivity compared to T1. These findings suggest that host-microbial interactions impacting host metabolism can occur during pregnancy and may be beneficial. The study highlights the profound alterations in the gut microbiota during pregnancy and the potential role of these changes in supporting fetal growth and preparing the body for lactation.This study investigates the remodeling of the gut microbiome and metabolic changes during pregnancy. The researchers characterized the fecal bacteria of 91 pregnant women with varying pre-pregnancy body mass indices (BMIs) and gestational diabetes status, as well as their infants. They found that similarities between infant and maternal microbiotas increased with the age of the child, and the infant microbiota was not affected by the mother's health status. The gut microbiota underwent significant changes from the first (T1) to the third (T3) trimesters, with a dramatic expansion of diversity between mothers, an overall increase in Proteobacteria and Actinobacteria, and reduced richness. T3 stool showed stronger signs of inflammation and energy loss, but the microbiome gene repertoires remained constant between trimesters. When transferred to germ-free mice, T3 microbiota induced greater adiposity and insulin insensitivity compared to T1. These findings suggest that host-microbial interactions impacting host metabolism can occur during pregnancy and may be beneficial. The study highlights the profound alterations in the gut microbiota during pregnancy and the potential role of these changes in supporting fetal growth and preparing the body for lactation.