A healthy fetal heart rate ranges between 110 and 160 beats per minute. A heart rate that is faster or slower than this may indicate that there is an issue that needs further attention.

An arrhythmia, or irregular heartbeat, is when the heart beats too quickly, too slowly, or with an irregular rhythm.

Fetal arrhythmia is an abnormal fetal heartbeat or rhythm. According to an article in the Indian Pacing and Electrophysiology Journal, the normal fetal heart rate ranges between 110 and 160 beats per minute (bpm).

Doctors diagnose fetal arrhythmias in 1–3% of pregnancies. While most fetal arrhythmias are benign, certain cases may require medical intervention.

This article explains how doctors diagnose a fetal arrhythmia, the different types, possible causes, and treatments.

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Doctors can detect fetal arrhythmias using ultrasound. If a doctor suspects an arrhythmia after reviewing a person’s routine ultrasound, they may recommend a fetal echocardiogram.

A fetal echocardiogram (fECG) is a safe and noninvasive test that allows a pediatric cardiologist to see the structures of the heart. The test can take anywhere between 45 and 120 minutes, depending on the complexity of the fetus’ heart.

Fetal magnetocardiography (fMCG) is not yet widely available. However, it can provide a more accurate picture of a fetus’ heart than fECG. Doctors can use fMCG to help diagnose heart arrhythmias in fetuses at around 20 weeks gestation and older.

A doctor may also order further tests, such as:

Sometimes, doctors cannot tell what is causing an arrhythmia. However, there are common causes, including:

  • problems with the heart’s electrical signals
  • structural abnormalities within the heart
  • congenital heart defects
  • restricted blood flow to the heart, or ischemia
  • electrolyte imbalance

There are many types of fetal arrhythmias. They include:

Extrasystoles or premature contractions (PCs)

The most common type of fetal arrhythmia is premature contractions or PCs. This arrhythmia happens when the fetus has extra heartbeats, or ectopic beats, that originate in the atria (PACs) or the ventricles (PVCs). PVCs are less common than PACs.

While most PACs are harmless and usually resolve over time, approximately 1% of fetuses with PACs will have significant structural heart disease. Another 0.5% will develop supraventricular tachycardia.

Fetal PVCs also usually resolve over time. However, doctors will monitor them closely as they may lead to proxysmal ventricular tachycardias (VTs).

Tachyarrhythmias

Fetal tachycardia refers to a heart rate faster than 180 bpm. It might occur when a pregnant person:

Fetal supraventricular tachycardia (SVT) is the most common type of fetal tachycardia. It occurs when the fetus’s heart rate is faster than 220 bpm. SVT complicates approximately 1 in 1,000 pregnancies and may lead to hydrops or heart failure.

Doctors may diagnose sinus tachycardia (ST) when a fetal heart rate is between 180 and 200 bpm.

Atrial tachycardia (AT) occurs when the heart’s upper chambers, the atria, beat too quickly. Ventricular tachycardia (VT) occurs when the heart’s lower chambers, the ventricles, beat too rapidly and cannot pump enough blood around the body. AT is more common than VT.

Bradyarrhythmias

Doctors may diagnose fetal bradycardia when a fetus’s heart rate is under 110 bpm for 10 minutes or longer. Persistent fetal bradycardia is relatively rare, and causes include:

  • congenital abnormalities
  • damage to the sinoatrial (SA) node
  • ion channel dysfunction, such as Long-QT syndrome
  • inflammation and fibrosis of the SA node
  • viral myocarditis
  • collagen vascular disorders
  • medications taken by the pregnant person, including beta-blockers and sedatives
  • rare metabolic disorders, such as Pompes disease

Approximately 30% of sustained bradycardia cases resolve without treatment.

Types of fetal bradycardia include:

  • sinus bradycardia
  • blocked atrial ectopic beats
  • atrioventricular (AV) heart block

Each case of fetal arrhythmia is different, and not all fetal arrhythmias require treatment. Many will resolve on their own.

Doctors prescribe treatment based on the cause of the fetal arrhythmia, a pregnant person’s health, the fetus’ health, and the pregnancy stage.

Medication is the most common way to treat a fetal arrhythmia. When a pregnant person takes medication, it passes through the placenta to the unborn baby. A pregnant person may take medications that include:

After childbirth, doctors will monitor a baby’s heart rate carefully and may recommend further medication.

Fetal arrhythmias are relatively common and account for 10–20% of referrals to fetal cardiology. The outlook for fetal arrhythmia depends greatly on the type and severity of the condition.

PCs is the most common type of fetal arrhythmia. It has a good prognosis and does not affect the growth and development of the fetus.

If doctors can make an accurate diagnosis of fetal arrhythmia, they can select the best treatments for a baby before and after its birth.

Sustained fetal arrhythmias can lead to hydrops, cardiac dysfunction, or fetal demise.

To find out more about fetal arrhythmia and heart conditions that affect children, including finding support groups, a person can ask a primary physician or cardiologist for recommendations. Alternatively, they can visit:

Most fetal arrhythmias are benign and resolve on their own. Doctors usually diagnose fetal arrhythmias using a fetal echocardiogram. However, they may also use other tests.

Doctors prescribe medication to treat fetal arrhythmias. They take various factors into account before choosing treatments, including maternal health and the severity of the condition.

Untreated fetal arrhythmias can lead to hydrops, cardiac dysfunction, or death of the unborn baby.

Doctors will monitor the health of the fetus and the pregnant person throughout pregnancy and after childbirth. A baby may require further treatment if the arrhythmia does not resolve on its own.