Jake was 9lb at birth and put on weight very quickly. I think I could have quite easily put him straight in medium Ittis and OWWs. He had one of each in a small and he looked like he was going to burst out of them at any moment
I also never bothered with small WNs, he was fine in mediums.
I did buy Teenyfits as I didn't know I was having such a big baby, they did fit but not for long and were a bit small in the rise really.
Size 1 Little Lambs are a good bet. They fitted Jake til about 2/3 months old, he outgrew the rise in the end x