Statistical analysis
We summarized descriptive characteristics of the study cohort using means with standard deviations and frequencies with percentages. We calculated descriptive characteristics for all births in the province >28 weeks to help describe the source population from which our tertiary care cohort was drawn.
We estimated the relationship between fetal size percentiles and EDI score using generalized additive modelling, which allowed us to describe associations using smooth, non-linear patterns in risk. All models included age at testing (range 4 to 7 years, mean = 5.6 years) as a covariate. We then categorized estimated fetal weight into five categories (<3rd, 3-9th, 10-19th, 20-30th, 31st-90th), and compared EDI scores in each category with the reference category of 31-90th percentile using absolute mean differences with 95% confidence intervals. We repeated these calculations for the two EDI subdomains, and for our two binary outcomes (using generalized additive modelling and predicting risks and risk ratios for these). For comparative purposes, we calculated EDI scores for all other children in the province born at or beyond 28 weeks, overall and in gestation age categories of 28-33, 34-36, and 37-42 weeks.
To evaluate the charts’ ability to predict developmental vulnerability at the individual level (i.e., discriminatory ability), we calculated the area under the receiver operating characteristic curve (AUC), as well as sensitivity, specificity, positive predictive value, negative predictive value, and likelihood ratios with 95% confidence intervals at select percentile cut-points of the chart (3rd, 10th, 20th percentile). Because the 10th percentile of both charts classified considerably less than 10 percent of our population as small-for-gestational age, we additionally identified the percentile on each chart that corresponded to the 10th percentile weight in our cohort, and evaluated predictive ability at this cut-off. Optimal thresholds for low and high risk were established using Youden’s Index.