Comment on “Maternal Fever During Labor and the Risk of Neonatal Encephalopathy: Duration and Magnitude of Hyperthermia”

Maternal fever in labor affects up to 1 in 10 deliveries and is associated with a higher risk of adverse neonatal outcomes, such as low Apgar scores, acidosis, respiratory distress, sepsis, seizures, and hypoxic-ischemic encephalopathy (HIE). However, it is unclear how the height and duration of fever in labor affect the risk of HIE. The aim of this study was to examine the association between maximal maternal temperature in labor and HIE and the duration from fever onset to delivery time on neonatal outcomes. This was a retrospective population-based cohort study, using data from 2012 to 2019 on all singleton live births ≥35 weeks in 15 Kaiser Permanente Northern California hospitals. Excluded were births involving elective cesarean delivery without labor, neonates with genetic or congenital anomalies, and deliveries with missing temperature data. Maternal fever was defined as having any recorded temperature ≥38 °C during birth admission between 72 hours before and 30 minutes after delivery. Maximal maternal temperature was categorized into 5 groups: <37.5 °C, 37.5-37.9 °C, 38.0-38.4 °C, 38.5-38.9 °C, and ≥39 °C. Fever duration was defined as the time between the first recorded maternal temperature ≥38 °C and delivery. The primary outcome was HIE. Secondary outcomes included low 5-minute Apgar score, acidosis, the need for therapeutic hypothermia, neonatal seizures, early-onset sepsis, and admission to the neonatal intensive care unit. A total of 248,594 mother-infant dyads were included in the analysis, with 10.4% experiencing a fever during labor. Of those with fever in labor, 487 delivered a neonate with HIE, a population incidence of 2.0 per 1000 infants. Mothers with a fever in labor were more likely to deliver neonates with HIE than those without fever [5.9 vs. 1.5 per 1000 births; risk ratio (RR), 3.92; 95% CI, 3.24-4.75]. The risk of HIE increased as the maximal maternal temperature increased. The lowest risk was in mothers with a maximal temperature <37.5 °C (1.4 per 1000), and the highest risk occurred in those with a maximal temperature ≥39 °C (11.5 per 1000). The ass`ociation continued after adjusting for labor duration. The risk of HIE also increased as fever duration increased. The lowest risk occurred in those with fever onset <1 hour to delivery (2.3 per 1000), and the highest risk occurred in those with fever onset ≥6 hours (10.5 per 1000). Infants exposed to fever <1 hour before delivery had half the risk of HIE than those exposed to fever from 1 to <3 hours. For infants exposed to fever ≥6 hours before delivery, the relative risk was 2.4 times greater (95% CI, 1.6-3.6). Infants exposed to fever for ≥6 hours had more than double the risk of HIE compared with those exposed for 1 to 3 hours (RR, 2.36, 95% CI, 1.55-3.60). After adjusting for labor duration, this association remained significant. In conclusion, the risk of HIE in offspring increased with the maximal maternal temperature and fever duration, even after adjusting for labor duration. (Summarized from Comet M-C, Kuzniewicz MW, Scheffler AW, et al. Maternal fever during labor and the risk of neonatal encephalopathy: duration and magnitude of hyperthermia. Am J Obstet Gynecol. 2025;233:651.e1-12. doi:10.1016/j.ajog.2025.07.046).