Measures
The primary outcome was impaired glucose tolerance at 1 year postpartum, defined as a HbA1c ≥5.7%. We evaluated the number of women with HbA1c ≥ 6.5%, consistent with type 2 diabetes, but did not consider type 2 diabetes as an outcome because of the small number of events. Covariate information for potential predictors of impaired glucose tolerance at 1-year postpartum were collected using surveys at study enrollment, in which women self-identified race-ethnicity, education history, and insurance status, and from electronic medical records, which included information on maternal age, weight and height, pre-pregnancy BMI, family history of type 2 diabetes, timing of GDM diagnosis and management, and delivery and neonatal complications. Information on plasma glucose values at 2 days postpartum was available for all study participants via study records.
We identified potential predictors known to be associated with progression from GDM to impaired glucose tolerance or type 2 diabetes postpartum based on literature review.21 Covariates were considered for inclusion into the predictive model if they were collected during prenatal care or within 2 days of delivery, in order to facilitate identification of women with recent GDM who may be at high risk for progression to impaired glucose tolerance prior to hospital discharge for delivery. Details about all candidate predictors considered are available in Supplemental Table S1. Potentially collinear candidate predictors were considered (e.g. pre-pregnancy BMI and obesity) to allow the model to select the covariate(s) with the best predictive value.22