A new study suggests it's dangerous and could potentially turn deadly.
According to data from the US National Center for Fatality Review and Prevention, 3 percent of the babies who died while sleeping over the past decade were in an "infant sitting device", such as a car seat, stroller, swing or infant seat.
63 percent of those deaths were in car seats, and 9 out of 10 of them weren't in the car.
Doctors urge parents not to use car seats as makeshift cribs. When they are outside the vehicle, babies should sleep in a crib or bassinet.
The data found that almost two-thirds of sleep-related infant deaths between 2004 and 2014 involved improper use of a sitting device.
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