When R-R intervals are short, the fetal heart rate is

Comprehensive and Detailed Explanation From Exact Extract NCC-Recommended Sources

The fetal heart rate is calculated from the interval between consecutive R waves in the fetal ECG. Shorter R-R intervals indicate more beats per unit of time, therefore resulting in a higher heart rate. AWHONN and Menihan both note that fetal ECG monitoring measures instantaneous rate based on R-R spacing, and “shorter intervals correspond to fetal tachycardia.”

Simpson & Creehan reinforce that fetal heart rate variability and baseline are derived from these R-R intervals, with shorter intervals consistently producing faster rates. Miller’s Pocket Guide describes the relationship simply: “Short R-R = faster rate; long R-R = slower rate.”