Objective <p>This study aimed to explore the influence of a simple increased adductor angle on gross motor development in infants, providing a theoretical basis for early identification and intervention.</p> Methods <p>A total of 1,132 infants aged 18 months or younger, born between July 1, 2020, and July 31, 2023, were enrolled. Participants were registered and prospectively followed up at the child healthcare department of a tertiary hospital. Basic demographic data and developmental milestone ages were collected from the hospital’s child healthcare information system. The adductor angle was assessed using the Infant Neurological Motor Assessment 20-Item Scale (INMA20) at corrected ages ranging from 28 days to 2 months for preterm infants and chronological ages within the same range for full-term infants. The cut-off ages for gross motor developmental milestones were established based on the Alberta Infant Motor Scale (AIMS) criteria.</p> Results <p>Chi-square analysis revealed a statistically significant difference in the incidence of a simple increased adductor angle between preterm and full-term infants (<i>P</i> &lt; 0.05). A logistic regression model with Firth correction indicated that a simple increased adductor angle had no significant effect on gross motor development outcomes in infants. However, the wide confidence intervals suggest that small effects cannot be completely excluded.</p> Conclusion <p>Preterm infants demonstrate a higher incidence of a simple increased adductor angle compared to full-term infants. However, this condition appears to have minimal impact on gross motor development in both preterm and full-term infants. Further research is warranted to establish a more accurate and developmentally appropriate normal range for the adductor angle during infancy.</p>