Fetal arrhythmia is most disconcerting because it is difficult to interpret arrhythmia in absence of fetal electrocardiogram (ECG) and we have to derive information from mechanic events of atria and ventricular. Atrial contraction represents P wave and ventricular contraction represents QRS complex and both should be seen simultaneously. This is achieved by M-mode and Doppler methods. M-mode is most useful in certain types of arrhythmia like atrial, ventricular tachycardia, and complete heart block (CHB) while Doppler in more helpful in ectopic beats or PR interval etc. Fetal arrhythmia is seen in 1–2% of unselected pregnancies. Methods to detect fetal arrhythmia and types of arrhythmias are covered in this chapter. Echocardiography is the most commonly used modality to diagnose fetal arrhythmia, other being magnetocardiography and fetal ECG. Magnetocardiography and fetal ECG are not commonly available.