What changes in the fetal heart rate are associated with neonatal acidosis during the second stage of labor?

OBJECTIVE

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

METHOD: 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.

RESULTS

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).

CONCLUSION

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.