The value of intrapartum factors in predicting maternal morbidity

The rates of severe maternal morbidity and mortality in the United States exceed those in other high-income nations. To aid providers and hospitals in recognizing the risk factors, there have been multiple attempts to develop stratification systems for morbidity based on maternal comorbidities. However, most women giving birth are healthy and do not have comorbidities to suggest that they are at an increased risk for severe maternal morbidity. There are small but inherent maternal risks to labor, and the events after admission may further influence a woman's risk for morbidity even for those initially at a low risk.

To determine if the incorporation of intrapartum factors known at the start of the second stage of labor improves the predictive performance of a comorbidity-based risk tool for severe maternal morbidity.

This is a retrospective cohort study of women at 8 hospitals in a single health system between July 1, 2016, and June 30, 2020. The women had term, singleton gestations and were admitted in labor and reached the second stage. The primary outcome was severe maternal morbidity. We compared logistic regression models using a validated risk-scoring tool (the Expanded Obstetric Comorbidity Score, which uses diagnosis codes for maternal comorbidities and pregnancy characteristics to predict maternal morbidity) with a model that included the Expanded Obstetric Comorbidity Score combined with parity and intrapartum factors. The intrapartum factors included labor induction or augmentation, length of labor, prolonged rupture of membranes, the presence of meconium-stained amniotic fluid, and gestational age. The hospitals were divided into a training (n=4) and testing (n=4) set to evaluate the predictive model performance. Discrimination was assessed by calculating the area under the receiver operating curve and calibration via calibration plots. Similar model comparisons were performed in a subgroup of women, who the Expanded Obstetric Comorbidity Score predicted to be at low risk for morbidity.

This analysis included 33,770 deliveries from the 8 hospitals; severe maternal morbidity occurred in 498 (1.5%) deliveries. The model performance is reported among the testing set (n=15,350). Using the Expanded Obstetric Comorbidity Score alone, the area under the receiver operating curve was 0.676 (95% confidence interval, 0.636-0.716) and 155 (71%) events occurred among individuals above the median predicted risk. When combining intrapartum factors, the area under the receiver operating curve increased to 0.729, (95% confidence interval, 0.693-0.764) and 171 (78%) events occurred among individuals above the median predicted risk. The significant factors that were associated with severe maternal morbidity in this combined model included the Expanded Obstetric Comorbidity Score, length of labor, and the presence of meconium-stained amniotic fluid. The area under the receiver operating curve for the model with intrapartum factors was significantly higher than the models using the Expanded Obstetric Comorbidity Score alone (P<.001).

The incorporation of intrapartum factors along with a validated risk tool (Expanded Obstetric Comorbidity Score) improved the ability to predict severe maternal morbidity at the start of the second stage. These findings emphasize the evolution of a woman's risk during her labor course and suggests that the prediction of maternal risk can be improved by considering intrapartum factors.