Gestational age and birthweight for risk assessment of neurodevelopmental impairment or death in extremely preterm infants

Abstract

Background: The risk of poor outcomes in preterm infants is primarily determined by birthweight (BW) and gestational age (GA). It is not known whether BW is a better outcome predictor than GA.

Objective: To test whether BW is better than GA (measured in days, rather than completed weeks) for prediction of neurodevelopmental impairment (NDI) and death.

Design/methods: Extremely preterm infants born at the National Institute of Child Health and Human Development (NICHD) Neonatal Research Network centres between 1998 and 2009 were studied. For the unadjusted analysis, the associations of GA (in days based on best obstetrical estimate) and BW (in grams) with NDI or death were compared using area under the curve (AUC). Adjusted analyses were performed using birth year, sex, race, antenatal steroids, singleton birth, pre-eclampsia, Apgar score at 5 min and small for GA as covariates.

Results: 10 652 preterm infants (89%) had outcome data at 18-22 months' corrected age. The mean BW was 678 g (SD: 155) and the mean GA was 173 days (SD: 10) or 245/7 weeks (SD: 13/7). The AUC for NDI or death was 80% with BW and 79% with GA (p=0.82). Unadjusted and adjusted analyses did not differ. NDI or death rates decreased with increasing GA through 26 weeks (estimated risk reduction with each additional day of gestation: 2.2%).

Conclusion: Both BW in grams and GA in days are good predictors of NDI and death in a preterm population selected on the basis of reliable GA.

Trial registration number: NCT00009633.

Keywords: extremely-low-birth-weight infants; extremely-low-gestational-age newborns; outcome prediction; premature infants; risk stratification.

Conflict of interest statement

Competing interests: None declared.

Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/.

Figures

Figure 1. Proportion of infants with NDI or death according to GA and BW

Panel A: Proportion of infants with NDI/death by BW NDI/death rate decreases with increasing BW over all values. Panel B: NDI/death rate decreases with increasing GA through 27 weeks (189 days).

Figure 2. Scatter plot of BW by GA to investigate collinearity

Each dot represents 10 infants grouped by similarities in both GA and BW. Values for proportions plotted are raw proportions for each BW and GA interval combination. For combinations with no subjects, occurring mostly at the extremes, the probability plotted is the value of the nearest cell (i.e. BW and GA combination). In most cases, this value is 1.