If baby shows any signs of unusual lethargy or unresponsive, vomitting (not spit ups.. Imagine something like Merlion projectile vomitting), bring baby to doctor asap.
However, to reiterate the point that another person has made, do not take baby out of the car seat to feed in the car. You can feed while baby is reclining in the car seat, and burp baby when you reach your destination. Been there, done that, children are absolutely fine with drinking milk while in the carseat and a slightly delayed burping session after the feed. There is increased risk of injury in car accident. I remember a report where the baby was flung out the car during a bad road accident where the mummy was holding baby to feed and baby died on the spot. Scared us enough that we all guai guai follow the rules after that.
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