Does anyone know what causes irregular fetal heartbeat?

Premature atrial CTX: >90% of all cardiac arrhythmias in fetuses are due to premature atrial contractions (benign) and resolve without intervention. They are more common with maternal caffeine consumption (tea, coffee, chocolate, coke/sodas) and smoking. The rest of the fetal arrhythmias are more serious but rare and include atrioventricular block, supraventricular tachycardia (paroxysmal or sustained), atrial flutter.