My LO used JB for a month before transiting to NB! But on the 3rd week or so, can tell the JB diapers are getting small on him. He was born at 36 weeks, but it rly depends on ur baby!
After the leftovers from hospital & free packet from insurance, switched to newborn on week 2 for mine.
Mine was using newborn frm born time.. my ger was 2.5.
before my girl turn first month we change to NB size.