I bought the Gerber onesies and I got these as well. The onesies run a bit small - the material is quite stretchable but tubby babies may have obvious bulging tummies. For these Sleep 'n Play, the sizes run somewhat big. I say that because I got onesies and sleep n play in the newborn size, and while my 7 week old probably can't wear the newborn onesies by next week, the newborn sleep and play is very baggy and I think she can wear it until she is 3-4 mths. Material is thin and comfy for warm weather. The zip only runs downwards so when changin nappy, her chest will be exposed. But better than those cursed pyjamas with buttons, which are a nightmare at 3am when your brain is half asleep and you are changing the nappy.