• The machine uses two round pads, called transducers, held on your belly with soft elastic belts.
• Heart rate sensor: One pad uses sound waves (Doppler ultrasound) to listen to the baby's heartbeat. A normal baby heart rate is between 110 and 160 beats per minute.
• Contraction sensor: The second pad uses a pressure monitor (tocodynamometer) placed on top of your belly to measure when and how hard your uterus tightens.