Birthweight and Patterns of Postnatal Weight Gain in Very and Extremely Preterm Babies: A 12 Year, Whole Population Study

Abstract

Background: Intrauterine and postnatal weight are widely regarded as biomarkers of fetal and neonatal wellbeing, but optimal weight gain following preterm birth is unknown. We describe changes over time in birthweight and postnatal weight gain in very and extremely preterm babies, in relation to major morbidity and healthy survival. Methods: We used whole population data from the UK National Neonatal Research Database for infants below 32 weeks gestation admitted to neonatal units in England and Wales over 2008-2019. We employed non-linear Gaussian process to estimate trends, and Bayesian multilevel regression to estimate unadjusted and adjusted coefficients. We evaluated birthweight; weight change birth to 14 days (ΔW14); weight at 36 weeks postmenstrual age (PMA); associated z-scores; and longitudinal weights for babies surviving to 36 weeks PMA with and without major morbidities. Findings: The cohort comprised 90,817 infants. Over the 12-year period mean (95% Compatibility Interval) differences adjusted for antenatal, postnatal and demographic variables were birthweight: 0g (-7, 7); birthweight z-score: -0∙01 (-0∙05, 0∙03); ΔW14: 39g (26, 51); ΔW14 z-score: 0∙14 (0∙08, 0∙19); weight 36 weeks PMA: 105g (81, 128); weight 36 weeks PMA z-score: 0∙27 (0∙21, 0∙33). Greater weight at 36 weeks PMA was robust to adjustment for enteral nutritional intake. In babies surviving without major morbidity, weight velocity in all gestational age groups stabilised at around 34 weeks PMA at 16-25 g/day along parallel centile lines. Interpretation: The birthweight of extremely and very preterm babies has remained stable over 12 years. Early postnatal weight loss decreased, subsequent weight gain increased but weight at 36 weeks PMA is consistently below birth centile. In babies without major morbidity, weight velocity follows a consistent trajectory offering opportunity to construct novel preterm growth curves despite lack of knowledge of optimal postnatal weight gain. Funding Information: UK Medical Research Council. Declaration of Interests: NM reports grants outside the submitted work from the Medical Research Council, National Institute of Health Research, March of Dimes, British Heart Foundation, HCA international, Health Data Research UK, Shire Pharmaceuticals, Chiesi Pharmaceuticals, Prolacta Life Sciences, and Westminster Children’s Research Fund; NM is a member of the Nestle Scientific Advisory Board and accepts no personal remuneration for this role. NM reports travel and accommodation reimbursements from Chiesi, Nestle and Shire. CG reports grants from the National Institute for Health Research and Canadian Institute for Health Research in support of randomised controlled trials of nutritional interventions in preterm neonates. All other authors report no declarations of interest. Ethics Approval Statement: The Health Research Authority, and Health and Care Research, Wales, approved the study (ID 273001). All neonatal units agreed to the inclusion of their data in the study.