Babies under 4 months old need to use the infant insert. This is how a newborn looks like with the insert. Take note of the placement of legs. For bigger babies, their legs can dangle outside. Maybe you can post a picture of how your baby looks like in the carrier with the insert and we can advise on that? Check out this video too.
https://www.youtube.com/watch?v=6SeOwBYCa3o
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