My son is 7 months but will be getting baptized towards the end of his 8th month so I opted for the 9-12 month size just to have some wiggle room for growth. It fits well, a little big but thats ok. I like the fact that the buttons are in the back of the shirt,very convinient for an on the go baby. The only thing I would have liked is if the shirt was actually a onesie. The shirt isn't long enough to stay tucked into his pants