To describe fetal heart rate (FHR) patterns associated with neonatal acidosis during the second stage of labor.

This single-center (Lille, France) retrospective cohort study was conducted between September 2022 and January 2023. Fetal heart rate (FHR) during maternal expulsive efforts was interpreted for all low-risk women who delivered vaginally after 37 weeks' gestation. Low risk was defined by the absence of any maternal history, obstetric complications, and a normal fetal heart rate in the hour before the onset of expulsive efforts. A baseline increase was defined as a greater than 20 % increase in baseline FHR. Reduced variability and marked variability were defined as a variability <5 bpm and greater than 25 bpm, respectively. FHRs were also described according to Melchior's classification. Neonatal acidosis was defined as umbilical artery pH <7.15.

Among the 275 expulsion FHRs analysed, 7.3 % (20/275) showed increased baseline and 5.5 % (15/275) abnormal variability. Neonatal acidosis occurred in 19.6 % (54/275) of neonates. Increased baseline and bradycardia were significantly associated with neonatal acidosis (14.8 % vs 5.4 %, p = 0.035 and 20.4 % vs 10.0 %, p = 0.035, respectively).

Increased baseline fetal heart rate during the second stage of labor was associated with a significant risk of neonatal acidosis. It would be interesting to include it in classifications related to this stage.