Fetal tachycardia is detected during pregnancy using a Non-Stress Test (NST). This condition is defined as fetal tachycardia when the fetus's heart rate exceeds 160 beats per minute during an NST. Various factors such as maternal fever, hyperthyroidism, anemia in the mother or fetus, the use of certain medications, and fetal heart anomalies can be among the potential causes of fetal tachycardia.