Quoting Elijah C's Mommy!:" What size did/does your 4 month old wear? My 4 month old wears size 3. We switched to size 3 when he ... [snip!] ... 15lbs. Edit : I asked this because my friends 10 month old DS JUST switched to size 3 and she said my babys not normal "
My son was only in newborns for about 3 weeks before he had to be in ones. Then he was in 1's until he was a month and a half. Once he was in two's though, he was in those for a 1 week lol. He's been in size 3's now since he was 2 1/2 months old and he'll be exactly 4 months this Friday.