DS1 was 6lbs 2.5 at 40w6d and wore premie.
DS2 was 7lbs 7.5 at 39w6d and wore NB and 0-3 was loose.
Depending on the brand (fits smaller or larger) it can vary, but I doubt average sizing 3-6 would do much short of swallow the kiddo whole. And unless your DH was 10+ I seriously doubt 3-6 much less 6-9. My 7mo can't even wear 6-9m yet, and he's probably around 13lbs now. My oldest was 20lbs at 6m and was just into 6-9m then.
I wouldn't go above 0-3 honestly unless the brand ran small.
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