Fetal monitors provide either external or internal sensing of the fetal heart rate and the associated contraction sequence while creating a permanent record of the data. It is used for observing and recording the heart rate of a fetus and for keeping track of the frequency, length, and strength of the mother's uterine contractions. Most external fetal monitors used a Doppler ultrasound transducer applied to the mother's abdomen. The transducer detects fetal heart activity and triggers the rate-computing circuitry.