Catch‐Up Weight Gain and Gut Microbiota Development in Full‐Term Small for Gestational Age Children During the First Year of Life—A Prospective Cohort Study

Objectives Gut microbiota develops dynamically during infancy in parallel with early growth processes. This study aimed to assess the pattern of gut microbiota colonization in full‐term SGA infants with catch‐up weight gain in the 1st year of life. Methods This longitudinal cohort study included 19 full‐term SGA and 46 full‐term appropriate‐for‐gestational‐age (AGA) infants. Stool samples and body mass measurements were collected at multiple time points during the 1st year of life. Gut microbiota composition was analyzed using 16S rRNA gene sequencing. Alpha diversity, beta diversity, and taxa abundances were used to evaluate microbial composition and diversity across developmental stages. Associations between the rate of weight gain and the pace of gut microbiota maturation were examined. Results SGA infants exhibited higher alpha diversity than AGA children at most time points. In this group, the Shannon index, reflecting the level of gut microbiota maturation, was positively associated with the rate of body weight gain over time ( p = 0.015), an association that was not observed in AGA infants. Characteristic genera associated with SGA included Citrobacter, Staphylococcus, Blautia, Veillonella, Klebsiella , and Clostridium XIVa . Conclusions SGA children had a distinct gut microbiota with higher alpha diversity than AGA peers. In this group, more mature microbiota was linked to faster weight gain and an increased abundance of short‐chain fatty acid–producing and obesity‐associated bacteria, suggesting that early microbial development may affect the risk of overweight and obesity later in life.