Longitudinal BMI growth curves for preterm infants internally validated

Body mass index (BMI) growth curves generated with longitudinal data for preterm infants demonstrated an initial decline in BMI driven by weight, followed by a linear increase in weight that was more pronounced in the lower gestational age (GA) groups.

Evidence Rating Level: 3 (Fair)

Study Rundown: Surveillance of growth in infants and children is an integral component of health assessment. Infants born prematurely are at increased risk of poor extrauterine growth compared to their term counterparts. The purpose of this study was to generate longitudinal growth curves of preterm infants for BMI. Using anthropometric data from a multi-center database spanning 33 states and 248 hospitals, the authors constructed BMI curves for extremely preterm (EPT, 24-27 weeks’ GA), very preterm (VPT, 28-31 weeks’ GA) and moderate-to-late preterm (32-36 weeks’ GA) infants. In validation analysis, the models demonstrated normality, goodness of fit, and good prediction performance. These longitudinally derived curves revealed a nadir in BMI during the first 2 weeks of life for all GA groups and both sexes, driven primarily by weight loss. The nadir was followed by a linear increase in BMI over time, with premature infants showing the greatest increase. Limitations of this study include reliance on data collected during routine clinical care, which are therefore predisposed to error and significant exclusion of data from the initial sample. For pediatricians, these curves will allow for identification of different growth patterns that may be correlated with short- and long-term outcomes for preterm infants.

Click to read the study, published today in Pediatrics

Relevant Reading: BMI curves for preterm infants

In-Depth [cross-sectional study]: Participants included 189 782 preterm infants between 24-36 weeks’ GA. Anthropometric data between 2009-2013 was collected through the Pediatrix Medical Group, Inc, which included 248 hospitals across 33 U.S. states. The final sample set included 68 693 (36%) infants from the original cohort, 25% and 75% of which were randomly split into a validation and curve creation set. Infants with major anomalies, death, transfer out of network, or missing sex/anthropometrics were excluded. Median growth curves were constructed using gestational day-specific z-scores from previously cross-sectional data. Generalize Additive Models for Location, Scale and Shape (GAMLSS) software was used to create the sex- and GA-specific BMI curves. Curves were terminated at 60 days, 45 days, and 30 days for EPT, VPT, and moderate-to-late preterm infants based on declining observations as infants approached discharge from the neonatal intensive care unit (NICU). Validation was demonstrated through normality, goodness of fit, and prediction of accuracy. For normality, plots of z-scores based on the validation data set did not show significant deviation from normality except for the first 3 days after birth. Comparison of the fitted median curves wit the empirical median curves showed goodness of fit. Of 9 randomly sampled infants, 7 had growth trajectories that followed predicted growth patterns. The longitudinal curves in this study detected an initial BMI nadir in the first 2 weeks after birth that was not detected in previously derived cross-sectional BMI curves.

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