I think you are right that newborns typically lose weight. But, if I remember right, they are supposed to return to their birthweight "by the time they leave the hospital" (so very quickly!).

Our son lost 7 ounces the first week.  I really don’t know if that is considered too much. I just remember his doctors being very concerned about his weight before the infant screening test came back.

He was born on a Monday night (c-section) and we were discharged on Friday.  They wanted him in for a weight check on the following Monday morning. By Tuesday they had the newborn screening results and he was started on hydrocortisone Tuesday night.

Hope that helps.

-Chris