Statistical analysis
Statistical analysis was performed with R Studio software (Version 1.3.959). Categorical variables were compared using χ2or Fisher’s exact test where appropriate, and continuous variables were compared using T-test. The Mann Whitney U Test was used to compare quantitative CGA measurements and their associations with the primary outcome. Further, the analogous continuous outcome of GA at delivery was assessed relative to the quantitative CGA measurements using Spearman’s correlations. The relationship between PTB, CL, and CGA absence was modeled using multivariate logistic regression via a stepwise backward elimination approach, controlling for variables identified as significant in univariate analysis or that were deemed clinically relevant for inclusion in the model. In constructing the model, Spearman’s correlation testing was used to make pair-wise comparisons between predictor variables to ensure absence of multicollinearity. Full and reduced versions of the final model, which differed only by inclusion of CGA evaluation with TVUS CL screening, were compared using the likelihood ratio test. Receiver operating curves (ROCs) were constructed to evaluate performance of the nested models (with and without evaluation of CGA at TVUS CL screening) for predicting PTB <37 weeks, and the areas under the curve (AUC) were determined. Statistical significance was defined as p<0.05.