Baby monitors have expanded from audio devices into products claiming physiological monitoring. What each type actually measures differs considerably from what buyers often assume.
Audio monitors transmit, they do not assess
A basic monitor is a one-way radio. It reproduces sound above a threshold and makes no judgement about what the sound means.
Sensitivity settings determine what gets through, and a setting low enough to avoid constant noise will also suppress quieter sounds.
Interference, range limits and battery failure all interrupt transmission without necessarily producing an obvious alert. Absence of sound is not confirmation that the link is working.
Video adds a viewpoint, not coverage
A camera shows one fixed angle. A baby who has moved outside that frame is not visible, and the display gives no indication that anything is missing.
Infrared illumination allows a picture in darkness, at reduced detail. Subtle changes in colour or breathing are not reliably visible in that mode.
Camera placement also introduces its own consideration, since cords near a cot are a recognised strangulation hazard and require routing well out of reach.
Movement sensors infer rather than measure
Under-mattress sensor pads detect motion transmitted through the mattress and interpret its absence as a lack of breathing movement.
They cannot distinguish a baby who has rolled to the edge of the pad from one who has stopped moving, which is a common source of alarms.
Conversely, external vibration from a washing machine, a passing vehicle or another person can be interpreted as movement. The inference runs in both directions.
Wearable sensors face the limits of the technique
Devices measuring oxygen saturation or pulse from a sock or band use the same optical principle as clinical equipment, in a consumer form.
Accuracy depends heavily on fit, movement and skin contact, all of which are less controlled at home than in a hospital. Both false alarms and missed readings occur.
Consumer devices are generally not regulated as medical equipment, which means their performance has not been assessed against clinical standards.
The behavioural effect is the underexamined part
A monitor that appears to confirm everything is fine can reduce the checking and supervision that would otherwise happen. The device substitutes for attention rather than adding to it.
Frequent false alarms produce the opposite problem, as repeated alerts that turn out to be nothing reduce the response to any alert.
No monitor is a substitute for established safe sleep practice, and any concern about a baby's breathing, colour or responsiveness is a matter for immediate medical assessment rather than a device reading.