How to know when to go up clothes size

Jennifer

My LO is 7.5 weeks and miraculously can still fill in NB but some of them are very ~fitted. I can pull on them and there’s some additional room but I don’t know when to make the final switch to 0-3 mo. Is it when the onesies are getting tighter like hers or when they literally don’t fit or there’s not extra room?