What could be the cause of a fast heart rate in a term fetus?

Depends.: If the fetal heart rate remains above 160 beats per minute for over 10 minutes, then fetal tachycardia is diagnosed. This can be due to maternal fever, or fetal hyperthyroidism, cardiac defect, circulatory compromise, excessive fetal activity (benign, transient), cardiac failure, or fetal tachyarrhythmia (atrial flutter, fibrillation, supraventricular tachycardia).