Our granddaughter is on the small side. She was one on Christmas day. The reviews said it ran large so I felt safe purchasing the 12 month.  Right now it fits great, but it will be too small within the next couple of months.  I love the suit.  It is easy to put on, she's covered and clearly comfortable. It dries quickly. I wash it, but I don't dry it.  I will probably purchase another one when she grows out of this one. I've had a couple of mothers stop and tell me they like the suit.