Fetal bradycardia refers to an abnormally low fetal heart rate, a potentially ominous finding. A sustained first trimester heart rate below 100 beats per minute (bpm) is generally considered bradycardic. The average fetal heart rate changes during pregnancy, however, and some consider the lower limit of normal at ²:

• 100 bpm up to 6.2 weeks of gestation
• 120 bpm at 6.3-7.0 weeks

Pathology

Fetal bradycardia can arise from a number of causes:

• underlying conduction abnormality
• following cordocentesis ^6,7
• vagal cardiovascular reflex (especially if transient during 2^nd trimester):
  • fetal head compression
  • umbilical cord occlusion/compression
  • maternal exertion: possibly from inadequate maternal gas exchange ⁵
  • hypoxia caused by myocardial depression
  • stimulation of the stretch receptors in aortic arch and/or carotid sinus walls

Classification

• fetal sinus bradycardia
• fetal bradyarrhythmia(s)
  • fetal atrioventricular block
    • fetal partial atrioventricular block (PAVB)
    • fetal complete atrioventricular block (CAVB): commonest type of bradyarrhythmia ¹¹
  • blocked premature atrial contractions

Associations

• increased risk of chromosomal anomalies ^9,10, especially trisomy 18
• maternal connective tissue disease: particularly with bradyarrhythmias

Treatment and prognosis

The lower the fetal heart rate, the worse the prognosis. Heart rates of <90 bpm in the first trimester are considered to have a dismal prognosis ^3,8.

• gestational age ≤6.2 weeks (crown-rump length ≤4 mm)
  • heart rate ≥100 bpm: 89% survival
  • heart rate 90-99 bpm: 68% survival
  • heart rate 80-89 bpm: 36% survival
  • heart rate <80 bpm: ~0% survival
• gestational age 6.3-7.0 weeks (crown-rump length 5-9 mm)
  • heart rate ≥120 bpm: 93% survival
  • heart rate 110-119 bpm: 87% survival
  • heart rate 100-109 bpm: 52% survival
  • heart rate <100 bpm: ~0% survival

Transfer to a tertiary center with cardiology support is often recommended.

Differential diagnosis

General considerations include:

• transient sinus bradycardia from excessive transducer pressure ⁴