Statistical analysis
As we stratified neonatal outcomes according to timing of delivery, we also compared how maternal characteristics, obstetric features, fetal characteristics, and circumstances at parturition varied with each completed week of gestation at the time of delivery. Categorical variables were presented as n (%) and tested with Chi square test or Fisher’s exact test as appropriate. Normally distributed continuous variables were presented as mean ± SD and compared with One Way ANOVA. Non-normally distributed continuous variables were presented as median (IQR) and tested with One Way ANOVA on ranks. A level of statistical significance of P ≤ 0.05 was considered
Multivariate logistic regression analysis was used to investigate if gestational age and circumstances at delivery (spontaneous PTL, pPROM or indicated delivery) independently affected the risk of adverse neonatal outcomes. The following variables were tested as potential confounders: maternal age, parity, history of spontaneous preterm birth, race, low education, smoking, maternal BMI, excessive weight gain, utilization of assisted reproductive technologies (ART), treatment with antenatal corticosteroids, ASA, LMWH, or progesterone. Maternal medical disorders, obstetric or fetal complications, type of labor other than spontaneous (i.e induced or no labor), and route of delivery were not included in the models as they were considered on the same causal pathway of circumstances at delivery. Finally, a potential interaction between gestational age at delivery and circumstances at delivery was investigated.
Disorders of the mother or the fetus may affect neonatal outcomes, even if they are not the primary indication for delivery. Therefore, we tested the independent effect on adverse neonatal outcomes of maternal medical conditions (diabetes mellitus, hypertensive disorders, liver disorders), fetal characteristics (non-reassuring fetal status, IUGR, prenatally diagnosed fetal anomalies or aneuploidies, and amniotic fluid disorders), pregnancy complications (pPROM, spontaneous PTL, chorioamnionitis, vaginal bleeding from placental abruption or abnormal placentation), and gestational age at delivery using multivariate logistic regression. Maternal age, parity, history of spontaneous preterm birth, race, low education, smoking, maternal BMI, excessive weight gain, utilization of ART, treatment with antenatal corticosteroids, ASA, LMWH, or progesterone were tested as potential confounders. Type of labor other than spontaneous (i.e induced or no labor), and route of delivery were not included in the models as they were considered on the same causal pathway of maternal, fetal, or obstetrics complications.
The strength of the association between the covariates and the dependent variable was estimated as area under the curve of a receiver operating characteristic (ROC) curve plotted with the true-positive rate compared with the false positive rate. Statistical analyses were performed using Stata 15 (StataCorp, College Station, TX).