External monitoring can be done by listening to your baby's heartbeat with a special stethoscope. More often, external monitoring is done using two flat devices (sensors) held in place with elastic belts on your belly. One sensor uses reflected sound waves (ultrasound) to keep track of your baby's heart rate. The other sensor measures the duration of your contractions. The sensors are connected to a machine that records the information. Your baby's heartbeat may be heard as a beeping sound or printed out on a chart. The frequency and duration of your uterine contractions are usually printed out on a chart.