Reference values for fetal heart rate monitoring in a large tertiary hospital population: a retrospective study

OBJECTIVE: To establish reference values for fetal heart rate (FHR) indices across time, frequency and nonlinear domains throughout pregnancy in a tertiary hospital population, considering sex. The influence of the number of fetuses, birth weight ,and time to delivery on FHR was evaluated. METHODS: This retrospective cohort study analyzed the initial FHR tracing upon hospital admission between 24° and 41° weeks of gestation, excluding cases in labor, with medication use, or a confirmed medical indication. Reference values were established using the Generalized Additive Models for Location Scale and Shape framework. Likelihood ratio test assessed whether including clinical variables significantly improved model fit. Birth weight was below p10 in 22% and above p90 in 9%. Median tracing duration was 42.5 min and median signal loss was 1.95%. Most indices were significantly associated with gestational age and several showed significant sex differences. Model fit significantly improved for multiple indices when including number of fetuses, birth weight, or time to delivery. CONCLUSIONS: This article presents gestational age- and sex-specific reference values for FHR in a large tertiary hospital population. The influence of gestational age was reaffirmed and significant differences related to sex, number of fetuses, birth weight, and time to delivery were identified. This enhances understanding of fetal autonomic regulation and supports a more individualized approach to predictive fetal monitoring. Further research is needed to determine the clinical utility of these reference values in practical monitoring and risk assessment.