Postnatal growth of etiologically characterized preterm newborns according to gestational age at birth.

Objective: To examine the relationship between etiologically-based preterm birth sub-groups and early postnatal growth according to gestational age at birth.

Methods: Prospective, multinational, cohort study involving 15 hospitals that monitored preterm newborns to hospital discharge. Measures/exposures: maternal demographics; etiologically-based preterm birth sub-groups; very, moderate and late preterm categories, and feeding.

Primary Outcomes: serial anthropometric measures expressed as z-scores of the INTERGROWTH-21 preterm postnatal growth standards.

Results: We included 2320 singletons and 1180 twins: very=24.4% (n = 856, including 178 < 28 weeks' gestation); moderate=16.9% (n = 592) and late preterm=58.6% (n = 2052). The median (interquartile range) postmenstrual age at the last measure was 37 (36-38) weeks. The 'no main condition' sub-group percentage increased from early to late preterm; the 'perinatal sepsis' sub-group percentage decreased. 'Perinatal sepsis', 'suspected IUGR' and 'fetal distress' very and late preterm infants had lower postnatal growth patterns than the 'no main condition' reference sub-group. This pattern persisted in late but not very preterm infants when postnatal growth was corrected for weight z-score at birth.

Conclusion: The proportional contribution of etiologically-based preterm sub-groups and their postnatal growth trajectories vary by preterm category. Postnatal growth is partially independent of fetal growth in the majority of preterm infants (i.e., those born late preterm).

Impact: Preterm birth, the leading cause of under-5 mortality, is a highly heterogenous syndrome, with surviving infants at risk of suboptimal growth, morbidity, and impaired neurodevelopment. Both the proportional contribution of etiologically-based sub-groups and their postnatal growth trajectories vary by preterm category (very/moderate/late). The 'perinatal sepsis', 'suspected IUGR' and 'fetal distress' sub-groups amongst very and late preterm infants had lower postnatal growth than the 'no main condition' preterm infants. The pattern persisted after adjusting for birth size only in the late preterms. Postnatal growth is partially independent of fetal growth in the majority of preterm infants (i.e., those born late preterm).