My 5 month old has been wearing this brand and type bodysuit from day 1.  The 0-3 month size still fits him at 5 months, and he's an extra long baby and average weight.  Bought a dozen of these in the 6-9 month size b/c they are so great.  Love how there is no tag to annoy baby's skin, the extra row of snaps for long babies, and the easy over the head neck line design.  They are great for layering when it's cold and my little guy sleeps in just these and a diaper when he goes to sleep in his sleep sack.  When the weather gets warmer, I think he'll just wear these all day around the house.  Couldn't be happier with this product.