Time to catch-up: postnatal growth trajectories in preterm infants at a tertiary care hospital in Riyadh, Saudi Arabia

BACKGROUND: Preterm birth remains a significant global health concern, with long-term outcomes closely linked to postnatal growth trajectories. Data regarding the timing of catch-up growth in preterm infants from Middle Eastern populations are limited. This study aimed to assess the time required to achieve catch-up weight and to characterize postnatal growth trajectories across birth-weight categories at a tertiary care hospital in Riyadh, Saudi Arabia. METHODS: This retrospective cohort study included 134 preterm infants (< 37 weeks' gestational age) and 193 term infants born between January 2018 and early 2019 at King Saud University Medical City. Anthropometric measurements were recorded at predefined intervals up to 24 months of corrected age. Growth was evaluated using sex-specific Z-scores based on WHO Child Growth Standards. Catch-up growth was defined as a Z-score increase > 0.67 between two consecutive time points. Longitudinal weight and length trajectories were analyzed using linear mixed-effects models with subject-specific random intercepts and a first-order autoregressive covariance structure, with adjusted estimated marginal means derived over time. Reporting followed the STROBE statement for cohort studies. RESULTS: Preterm infants had significantly lower birth weight (1.72 ± 0.60 kg vs. 2.97 ± 0.66 kg; p < 0.001) and birth length (40.65 ± 5.36 cm vs. 48.27 ± 3.55 cm; p < 0.001) compared with term infants. In mixed-effects models, preterm infants had lower adjusted weight overall (p = 0.006), but the group-by-age interaction was not significant (p = 0.271), indicating parallel weight gain over time. For length, a significant group-by-age interaction (p < 0.001) indicated that linear-growth trajectories differed between groups, with preterm infants remaining shorter throughout follow-up. Weight-for-age Z-scores converged across birth-weight categories by 24 months. Catch-up growth rates were highest among very low birth weight infants (31.8% by 24 months). CONCLUSIONS: Preterm infants demonstrate significant early growth deficits with weight catch-up achieved within the first year of corrected age, while linear growth recovery extends closer to 24 months. These findings underscore the importance of vigilant growth monitoring and individualized nutritional strategies during early infancy.