My 10-week old girl fits perfectly in the 3-6 month size right now (on the smaller set of snaps).  She is very tall though and generally into 6-months in most brands.  The onesies are nice and soft and I love the paris motif (we honeymooned there).  I love that they are "expandable" as she continues to grow a bit since she is growing out of things very fast!  I will probably buy these or another pattern in a larger size next since we liked them.