this swimsuit\/diaper be really nice . it be so thin , and fit well . I buy a medium for my son , who weigh 18 lb , and it fit great . I also buy a small for a friend who be have a baby , and it look perfect for a 7-12 lb baby . it be very cute , and seem very well-made . you do not need a separate diaper insert , or cover , or anything - it be all one piece .