Simon Cowell took his newborn son Eric to the doctor by car for a check up in New York. It's been frigid in New York, so baby Eric is warm and snug in his Cybex Aton Infant Car Seat. The issue we have (yes, Simon we're picking a bone with you) is that Eric is in a sleeping bag made by a different manufacturer than his car seat. That means there's a blanket under him and under his straps. The car seat manufacturer's manual clearly prohibits the use of an item like this and any tech will explain that the sleeping bag can interfere with the functionality of the straps and the amount of slack in the straps in an accident. It's scary to think about these things, but it's important. Clearly Simon Cowell had no idea about this safety issue, but hopefully someone fills him in.
There are many alternatives to keeping baby out of the cold winter air without adding additional items to an infant car seat that can potentially affect the safety of that seat. One would be a simple blanket on top of the baby. If you're looking for an actual product, try something that goes over the seat "shower cap style". This will not interfere with the straps or the seat's safety.
At NessaLee Baby we have CPSTs on staff because car seat safety is something we are truly passionate about. If you have questions contact us and we're happy to help.