More than the passage of time: neonatal health and active labor duration, the mediating role of chorioamnionitis

Background: Labor intervention is intended to prevent perinatal morbidity and mortality. Labor durations traditionally defined as 'dystocia' drive primary cesarean birth rates, yet there is mixed evidence about the association between labor duration and poor neonatal outcomes. Using latent class analysis to study unknown patterns of neonatal health can refine understanding of these relationships and effect of labor duration on newborn outcomes.

Objective: The purpose of this study was to 1) identify latent classes of newborn health characteristics, 2) describe pregnancy and labor characteristics of individuals in the classes, including active labor duration, 3) determine probability of class membership (newborn health) using labor duration and associated factors as predictors, and 4) assess direct and indirect effects of labor duration versus chorioamnionitis on neonatal risk.

Study design: Latent class analysis of 3779 term, cephalic, singleton and non-anomalous maternal/child dyads using prospectively collected, observational data.

Results: Latent class analysis of 3779 term, cephalic, singleton and non-anomalous maternal/child dyads identified 6 neonatal phenotype classes: 4 (classes 1-4, 92% of the data) with normal neonatal outcomes and 2 (classes 5 and 6) with abnormal neonatal outcomes. Apgar scores were not related to more severe markers of neonatal well-being [NICU admission; morbidity diagnosis; failure to discharge home when parent was discharged]. While median labor duration was longer with classes 5 and 6, the majority had labors in the lower three quartiles. Most births in the longest quartile of labor were associated with healthy newborn outcomes. Chorioamnionitis mediated 26% of the relationship between labor duration and newborn health phenotype.

Conclusion: Neonatal risk may be driven or mediated by several labor and birth factors, many of which are challenging to disaggregate. Prior research presuming longer labor duration as the driver of neonatal risk may have masked other relevant associations. More research regarding physiologic versus pathologic inflammatory and infectious processes during term labor is critical for advancing labor dystocia research and refining intrapartum management. Additional variables strongly correlated with labor dystocia (e.g., fetal malposition) should be examined as potential mediators between longer labor and poor neonatal outcomes.