You might see monitors on the market with claims that they can track a baby's breathing or movements, but unless the unit is registered with the FDA, it's not a medical device. Consumer Reports hasn't tested this type of monitor. Talk with your pediatrician if you think your child has a condition that warrants medical monitoring. He or she can give you advice on the best devices.
In terms of bang for your buck, it’s tough to beat the Babysense. For less than $100, it comes loaded with bells and whistles more commonly found on higher priced models. Not only does it boast a 2.4-inch HD LCD color screen with infrared night vision, pan/tilt, and 2x zoom, but it includes two-way talk back, a sound activated ‘Eco’ mode (the screen stays off) to save battery life, 900 foot range (with out-of-range warning), and an in-room temperature monitor that sends alerts if it gets too hot or cold. It uses 2.4 GHz FHSS wireless transmission for security and even comes with both a built-in alarm/nap timer and lullabies.
Image and audio quality: We wanted a high enough resolution to make out facial features in the dark, at more than a few feet of distance, and (obviously) in daylight as well. The screen itself did not need to be incredibly high-resolution, but we wanted a size that would be easily visible on a nightstand. For all monitors, but especially audio-only options, we wanted to be able to hear everything clearly at the lowest volumes.
There are two basic types: audio and video/audio. Some are analog, others are digital. All monitors operate within a selected radio frequency band to send sound from a baby's room to a receiver in another room. Each monitor consists of a transmitter (the child/nursery unit) and one or more receivers. Prices range from about $25 to $150 for audio monitors and about $80 to $300 for audio/video monitors.