Fetal bradycardia is a type of irregular heartbeat known as fetal arrhythmia. Fetal bradycardia occurs when a fetus (developing baby) has a sustained heart rate slower than 110 beats per minute (BPM).

Fetal bradycardia is rare. It’s often temporary and harmless. Persistent fetal bradycardia—bradycardia that doesn’t go away—may require treatment before and after birth.

Fetal Bradycardia Types

There are four types of fetal bradycardia.

Sinus Bradycardia

Sinus bradycardia occurs when a fetal heart rate stays below 110 BPM, despite the normal function of the heart’s electrical system.

Atrioventricular (AV) Block (First degree)

Atrioventricular block occurs when the electrical signals between the heart’s upper chambers (atria) and lower chambers (ventricles) last longer than they should. This slows the heart rate.

Partial Heart Block (Second Degree)

Partial heart block occurs when electrical signals from the heart’s top chambers (atria) are intermittently blocked from reaching the lower chambers (ventricles).

Complete Heart Block (Third Degree)

Complete heart block occurs when electrical signals from the heart’s top chambers (atria) never reach the lower chambers (ventricles). When this happens, the chambers beat separately. Without electrical signaling between the chambers, the heart rate slows.

Fetal Bradycardia Causes

The causes of fetal bradycardia vary by type.

Sinus bradycardia may be caused by a delayed signal to the heart muscle. This is due to a problem with the heart’s sinus node. Causes include:

  • Fetal distress.
  • Maternal autoimmune diseases such as lupus erythematous or Sjogren’s syndrome.
  • Maternal medications.

AV block is caused by a problem with the heart’s electrical system. Fetal AV block is more common in women with autoimmune diseases. A fetus with AV block may be at risk for developing a complete heart block.

Partial heart block is caused by a problem with the electrical signal between the heart’s upper and lower chambers.

Complete heart block is caused when no electrical signals connect the upper and lower heart chambers. Complete heart block is more common in fetuses of women with autoimmune diseases. This type of fetal bradycardia can lead to a dangerous form of fluid buildup called hydrops fetalis.

Fetal Bradycardia Treatment

Most types of fetal bradycardia resolve on their own and don’t require treatment.

Mild sinus bradycardia, AV block and partial heart block may only require monitoring.

More severe types of fetal bradycardia may require treatment or intervention. Fetal bradycardia interventions include:

  • Maternal blood work (to test for autoimmune disorders).
  • Routine fetal heart imaging (echocardiogram).
  • Medication.
  • Early delivery.

Complete heart block requires close monitoring. These babies may require treatment with medication to try to speed up their heart rate. Babies with complete heart block and heart rates lower than 70 BPM may need a pacemaker. A pacemaker is a device placed in the body to control heart rate. If a pacemaker is needed, it would be placed after the baby is born.