Fetal Heart Tones (FHT): Monitoring, Normal Ranges, and Clinical Interpretation

Fetal Heart Tones (FHT) — also documented as Fetal Heart Rate (FHR) — are the audible or electronically recorded heartbeats of the fetus. They are the primary real-time indicator of fetal well-being during pregnancy and labor.

Normal FHR Ranges by Gestation

Gestational Age	Normal Range (bpm)	Notes
6–7 weeks (transvaginal US)	90–110	Rises ~3 bpm/day
8–10 weeks	140–170	Peak rate
12 weeks (Doppler audible)	120–160	Baseline established
20 weeks	140–160	Mid-gestation
Term (≥37 weeks)	110–160	Baseline for labor monitoring

Monitoring Modalities

• Intermittent Auscultation (IA): Doppler or fetoscope q15–30 min (low-risk labor).
• Continuous Electronic Fetal Monitoring (EFM): External (tocodynamometer + ultrasound) or internal (fetal scalp electrode + IUPC); high-risk labor, induction, epidural.

Key Tracing Components (NICHD Terminology)

Feature	Definition	Normal
Baseline FHR	Mean rate over 10 min (excluding accelerations/decelerations)	110–160 bpm
Variability	Fluctuations in baseline	Moderate (6–25 bpm)
Accelerations	Abrupt ↑ ≥15 bpm × ≥15 sec (≥32 wks)	Present = reassuring
Decelerations	Abrupt ↓ ≥15 bpm × ≥15 sec	None (early) or variable (cord compression)

Category Interpretation (ACOG)

• Category I (Normal): Baseline 110–160, moderate variability, accelerations present, no late/decelerations → continue monitoring.
• Category II (Indeterminate): Any tracing not I or III → evaluate, correct reversible causes, consider intrauterine resuscitation.
• Category III (Abnormal): Absent variability + recurrent late decels / variable decels / bradycardia → immediate intervention (often cesarean).

Clinical Pearls

• Fetal tachycardia (>160): Maternal fever, infection, hyperthyroidism, medications, anemia.
• Fetal bradycardia (<110): Cord compression, uteroplacental insufficiency, maternal hypotension, rapid descent.
• Loss of variability: CNS depression (meds), acidosis, fetal sleep cycle (usually <40 min).

Accurate FHT interpretation saves lives; it requires training, standardized terminology, and clinical correlation.