While it may seem that your baby is perfectly still when she is sleeping, she really isn't. Our Movement Sensor with Sound Monitor detects your baby’s slightest movements, even while she sleeps. The under-the-mattress Sensor Pad detects your baby's slightest movements and sends the signal to the Nursery Unit, which sounds an alarm ONLY if no movement is detect for 20 seconds. It also transmits all other nursery sounds and has a Movement Indicator Light showing if movement is being detected. The Parents’ Units receive the alarm and all other nursery sounds. This Angelcare® monitor has a wide variety of optional features so you can customize it to suit your personal needs. The Angelcare® Movement Sensor will give you the peace of mind you won't get from just a sound monitor or even a video monitor. And, this monitor is an excellent value. It is 2 monitors in 1 so you don't have to purchase a sound monitor.