Association Between Marked Fetal Heart Rate Variability and Neonatal Acidosis: A Prospective Cohort Study

ABSTRACT Monitoring of fetal heart rate (FHR) is an important indicator of fetal well-being during labor, but it is subject to low specificity and interpretation variability; this causes the clinical effectiveness for preventing adverse outcomes to remain controversial. Fetal heart rate has inherent variability, but reduced or marked variability can reflect poor autonomic activity, indicating conditions such as fetal acidosis. Recent research has highlighted that marked variability can indicate fetal distress; more research is needed to validate and investigate this claim. This study aimed to understand the association between marked FHR variability during labor and subsequent neonatal acidosis. Data were collected from 2 French maternity units; inclusion criteria extended to women at 37 weeks' gestation or greater, with continuous FHR monitoring during labor. Exclusion criteria were intrauterine fetal death, medical termination, multiple pregnancies, noncephalic presentation, and cesarean delivery. The primary outcome for this study was neonatal acidosis, or an umbilical artery pH of less than or equal to 7.10. Secondary outcomes included severe acidosis (pH ≤7.0), a 5-minute Apgar score of less than 7, respiratory distress, neonatal intensive care unit admission, neonatal infection, and neonatal death. Final analysis included 4394 women who gave birth between January 1 and December 31, 2019. Of this population, 177 neonates experienced marked variability in FHR within 1 hour before delivery, with a median duration of marked variability of 2 minutes. Prevalence of neonatal acidosis was 15.3% in neonates who experienced marked FHR variability and 5.6% in those who did not (aRR, 2.30; 95% confidence interval [CI], 1.53–3.44). In addition, those with marked FHR variability more often experienced respiratory distress (aRR, 1.73; 95% CI, 1.15–2.58). Analysis was subsequently performed according to the National Institute of Child Health and Human Development category of FHR; the association between FHR variability and neonatal acidosis was significant in category I (aRR, 5.48; 95% CI, 1.88–15.96) and category II (aRR, 2.29; 95% CI, 1.40–3.74) groups. Category III FHR, however, had no significant association. Strengths of this study include the prospective cohort design, as well as generalizability. Subcategory assessment of the FHR patterns allowed for more accurate examination of the risk of neonatal acidosis. Limitations include an observational design and the strict exclusion criteria required to minimize bias, which reduces generalizability to low-risk groups. The findings of this study are consistent with previous literature in that FHR variability can be a significant marker for fetal outcomes. Clinically, these results can help providers recognize the risk of fetal and neonatal acidosis. Further research is needed to delineate further associations and assess interpretations of pattern variability.