Postnatal Growth Patterns in Children Born at Term and Preterm in a Large Primary Care Network

To characterize infant and toddler longitudinal growth patterns by gestational age groups through 36 months of chronologic age.

This is a retrospective cohort study of children seen in a Massachusetts statewide primary care network from 2017 to 2023, who had 5 or more growth measurements recorded, with at least 1 in the first 6 months of life. Growth measurements were converted to z-scores and analyzed at standardized well-child check intervals. Percentiles were calculated and time to surpass the -2 z-score threshold was assessed.

60,966 unique children and 1,545,254 growth measurements were included in this study. Infants born preterm demonstrated lower z-scores across all growth metrics compared with term infants at birth, and these differences were correlated with gestational age. Over time, growth parameters of infants born preterm approximated those of full term infants. Of infants born extremely preterm, <95% surpassed the -2 z-score for weight and height by 36 months, whereas 95% of infants born moderate and late preterm passed this threshold across all growth measurements by 24 months.

Infant and toddler growth varied by gestational age over the first 3 years, with infants born at the most immature gestational ages showing incomplete catch-up growth. Our findings support accounting for gestational age through 36 months for infants born at <32 weeks gestation, and through 24 months for those born 32-37 weeks gestation. These insights may inform updates to growth monitoring guidelines.