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.
If you're looking for a smartphone-compatible baby monitor that's designed first and foremost to fill that role, Wirecutter calls the iBaby M6S (Est. $135) "the least bad Wi-Fi monitor (so far)," simply because it's "slightly easier to set up than its competitors." Once again, however, experts disagree: Baby Bargains says Nest Cam "runs circles around iBaby when it comes to set up and ease of use."
It really depends on what you feel most comfortable with. There are audio monitors that allow you to listen to any noise coming from the nursery, vital monitors that track sleep and breathing and video monitors that add sight to sound. Babylist parents overwhelmingly choose video monitors. The security of seeing what your child is up to—like if they’ve gotten tangled in their swaddle, pulled their diaper off or climbed out of the crib—can be worth the extra cost of a video monitor.