46 - Length of labor and severe maternal morbidity in the NTSV population
Friday, January 29, 2021
4:00 PM – 4:15 PM EST
Objective: To study the relationship between length of labor and the risk of severe maternal morbidity (SMM) using generalized additive models (GAMs).
Study Design: This retrospective cohort study included nulliparous term singleton vertex (NTSV) births in 17 US hospitals (2016-2019). Clinical data were abstracted from medical records. SMM included blood transfusion, DIC, hysterectomy, eclampsia, venous thromboembolism, and amniotic fluid embolism. Cesareans performed without labor were excluded. A multivariable GAM was trained to predict SMM from 40 demographic, pregnancy and labor characteristics. GAMs are an extension of logistic regression that allow non-linear effects of continuous variables and aid interpretability by plotting risk for each variable independently while correcting for other variables in the model.
Results: The cohort included 32,203 births. The rate of SMM was 1.2% (N=392). Characteristics of the study population are in Table 1. The strongest predictors of SMM were birthweight, time from admission to complete dilation, maternal height, pre-eclampsia/gestational hypertension, and 2nd stage duration. Induction was not a strong predictor of SMM when other variables (including cervical dilation on admission) were accounted for. SMM risk started to increase when admission to complete dilation duration was 12 hours and continued to increase smoothly up to 30 hours (Figure 1a). In contrast, SMM risk increased abruptly when 2nd stage duration reached 4 hours, a clinically relevant cut-off for prolonged 2nd stage in nulliparas (Figure 1b). Unlike the smooth increase in SMM during the first stage of labor, this abrupt increase likely reflects the impact of medical intervention.
Conclusion: The relationship between length of labor and SMM in the NTSV population is non-linear. After accounting for other variables in the model, SMM starts to increase when the time from admission to complete dilation reaches 12 hours. In the 2nd stage, SMM increases abruptly at 4 hours, a clinically actionable cut-off for prolonged 2nd stage, and likely reflects the impact of operative delivery.