What does a fast heart rate indicate in a fetus?

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