I never used NBs with my dd so I'm not sure on fit there, but in sure it will all depend on size at birth.
What size did you order? I know my lo was out of 0s/smalls by 3-3 1/2 months and then she was in 1s/mediums for like a year. They are the closest to OS diapers of other brands. I had a SUPER chunky kid though lol so I started using 2s/larges at like 6 months old just so they would last til we were out of dipes :)
I bet they'll fit your lo until potty learning :) my daughter was 17lbs by like 4-5 months old! Lol