Most patterns will have a size chart for you to compare your baby's measurements to. Depending upon the item you will need to measure their chest or hips generally. For example, for a dress you usually will measure their chest. For pants, shorts, skirt, etc their hips. Unfortunately you can't just assume that a pattern for a 12 month old will fit your 12 month old....that would be nice though. But if you get into the habit of measuring....it will really save you a lot of time. You can also measure the actual pattern pieces less seam allowances and then compare that to your child's measurements....just to double check. I hope that helps you some. If not, ask again....and I'll try again.
Cindy
www.sew-blessed.blogspot.com