Normal Fetal Heart Rate Ranges in Late Pregnancy
The normal fetal heart rate (FHR) in late pregnancy ranges from 110 to 160 beats per minute (bpm), characterized by moderate variability of 6-25 bpm, presence of accelerations, and absence of decelerations. 1
Understanding Normal Fetal Heart Rate Parameters
Baseline Rate
- Normal baseline FHR: 110-160 bpm 1, 2, 3
- Some guidelines suggest a narrower range of 120-160 bpm based on computerized analysis of cardiotocography tracings 4
- The National Institute of Child Health and Human Development (NICHD) classifies a baseline rate of 110-160 bpm as a Category I pattern, indicating normal fetal acid-base status and well-being 1
Variability
- Moderate variability: 6-25 bpm is considered normal 1, 2
- Minimal variability: <6 bpm (potentially concerning)
- Absent variability: not visible (concerning)
Accelerations
- Presence of accelerations is a reassuring sign of fetal well-being
- For pregnancies ≥32 weeks: An acceleration is defined as an increase in FHR of 15 beats/minute above baseline lasting for 15 seconds 5
- For pregnancies <32 weeks: An acceleration is defined as 10 beats/minute above baseline lasting for 10 seconds, reflecting differences related to prematurity 5
Age-Related Considerations
Gestational Age Effects
- FHR decreases slightly during gestation 4
- At or beyond 40 weeks gestation, a baseline FHR between 150-160 bpm has been associated with higher incidence of:
- Meconium-stained amniotic fluid
- Maternal intrapartum hyperpyrexia
- Cesarean section for suspected fetal distress
- Lower Apgar scores
- Neonatal acidemia 6
Monitoring Methods
Non-Stress Test (NST)
- NST is considered reactive if there are two or more fetal heart rate accelerations during 20 minutes of observation
- NST is considered nonreactive if after 40 minutes one or no acceleration is detected 5
Biophysical Profile (BPP)
- Combines NST with ultrasound assessment of:
- Fetal breathing movements
- Fetal body movements
- Fetal tone
- Amniotic fluid volume 5
Abnormal Patterns to Recognize
Tachycardia
- FHR >160 bpm for ≥10 minutes 2
- May indicate fetal hypoxemia, infection, or other stressors
Bradycardia
- FHR <110 bpm for ≥10 minutes 2
- May indicate fetal distress requiring immediate attention
Decelerations
- Early decelerations: Generally benign, mirror uterine contractions
- Variable decelerations: Usually benign but can become concerning with atypical features
- Late decelerations: More concerning pattern requiring attention and potential intervention
- Prolonged decelerations: Last >2 minutes but <10 minutes, require immediate attention 1
Clinical Implications
- A normal FHR (110-160 bpm) with moderate variability and accelerations indicates good fetal oxygenation and acid-base status
- Abnormal patterns may require interventions such as changing maternal position, administering oxygen, or expediting delivery
- Proper documentation of FHR patterns and interventions is essential for clinical care and to minimize litigation risk 1
Understanding normal fetal heart rate parameters is crucial for accurate interpretation of fetal monitoring and appropriate clinical decision-making during late pregnancy.