'Average' Baby Size Charts

I googled 'average baby sizes' to get an idea of size for a knitted jacket I'm about to start for my GD. This is what I found:

18" chest measurement fits:
a newborn (one site);
3 months (one site);
3-6 months (3 sites);
6-12 months (2 sites);
9 months (2 sites);
12 months (1 site);
12-18 months (1 site).

I measured her