Background <p>In utero insults such as chorioamnionitis are associated with adverse outcomes. This study aims to examine the association between chorioamnionitis and fat mass (FM) in very preterm infants.</p> Methods <p>We conducted a retrospective cohort study of mother-infant dyads born &lt;32^6/7 weeks of gestation. Infant FM accretion was measured using air displacement plethysmography at term-equivalent age. Histological chorioamnionitis severity was staged based on placental pathology and included maternal/chorion-amnion inflammatory response (MIR) and fetal/umbilical cord inflammatory response (FIR). The association between chorioamnionitis severity and FM accretion was analyzed using linear regression models and mediation analyses.</p> Results <p>Among 375 mother-infant dyads analyzed, 104 (28%) dyads had MIR. FIR was found in 85 dyads with MIR (82%). Infants without MIR had lower FM in Kg and lower FM z scores than those with MIR (<i>p</i> = 0.0001). Infants with severe MIR had higher body fat percentages (Stage 3: 18% vs Stage 1: 14%, <i>p</i> &lt; 0.0001). There were no significant differences in other anthropometric growth rates. Gestational age partially mediated this association (49%).</p> Conclusion <p>Severe histological chorioamnionitis is associated with greater FM accretion at term-equivalent age, independent of gestational age. Without long-term data, it remains unclear whether this early-onset effect is transient or persists into later childhood.</p> Impact statement <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Chorioamnionitis is common in infants born preterm and is strongly associated with preterm birth.</p> </ItemContent> <ItemContent> <p>Infants born preterm exposed to chorioamnionitis have an increased risk of abnormal fat mass accretion at term equivalent age.</p> </ItemContent> <ItemContent> <p>Accounting for the severity of chorioamnionitis could improve the interpretation of body fat accretion in infants born preterm.</p> </ItemContent> </UnorderedList></p>