The INTERGROWTH-21^st Preterm Postnatal Growth Standards are robust across geographical settings and time

We investigated the robustness of the INTERGROWTH-21^st Preterm Postnatal Growth Standards for monitoring the postnatal growth of preterm infants a decade after the original study that produced them. The INTERPRACTICE-21^st Study assessed the early postnatal growth of all infants born preterm in 15 study sites across 14 countries. From this sample, we selected a 'standard-like' cohort of mother-newborn dyads as in the original study. Thus, we selected all singleton babies of healthy mothers, born between 27 and < 37 weeks' gestation, without congenital malformations, fetal growth restriction or severe postnatal morbidity, i.e. confirming their eligibility for inclusion in the original standards. We used, across all study sites, the same standardised anthropometric measurements and feeding practices based on human milk. The main outcomes were the skeletal, fat-free mass indicators head circumference and length, following WHO recommendations. Variance component analysis and second-degree fractional polynomial regression models in a multi-level framework accounting for repeated measures were used to compare the growth trajectory of the 'standard-like' cohort to the standards. Of 3634 preterm babies enrolled, 1998 were singletons with complete birth anthropometric data. Among these, 1073 (54%) mother-baby dyads met the 'standard-like' eligibility criteria. The between-site differences in early postnatal growth of head circumference and length accounted for only 10.2% and 12.7% of the total variability of these repeated measures, proportions considerably lower than those for the total between individuals within a site variance of 80% and 79%, respectively. These low proportions supported pooling data across study sites and reinforced the consistency of the original results across geographical settings.

The early growth of healthy preterm infants in the INTERPRACTICE-21^st 'standard-like' cohort, assessed a decade after the original study, closely matched the pattern originally described by the INTERGROWTH-21^st standards, which appear therefore robust to geographical changes and time.

• The INTERGROWTH-21st Preterm Postnatal Growth Standards are the only international standards, based on the WHO prescriptive approach, for monitoring the growth of preterm infants. They were produced by following, from early pregnancy, uncomplicated preterm infants who were born to healthy mothers and achieved adequate growth and development at age 2.

•  In a study conducted in 14 countries worldwide, skeletal growth trajectories in a contemporary "standard-like" cohort of preterm infants demonstrated much larger within, than between, study site variance and closely matched the INTERGROWTH-21st standards, supporting the consistency of preterm growth across populations and over time.