All newborn babies loose some weight in the first few days. They come from the water environment of mom's uterus to the air environment outside. It is like picking up a sponge that has been in the dishwater — the extra water drips out for a while.
About three days after birth, just about the time mom's milk comes in, the baby has peed out all the extra fluid. We expect her to start gaining weight a little at a time, so that by two weeks of age, she should be back to, or over, her birth weight.
Now, we don't want that little baby "sponge" to get too dried out (dehydrated), so she gets weighed every night. A weight loss of six to seven percent of birth weight is considered normal. If her daily (or nightly) weight shows a loss of eight to ten percent, or more, we start getting worried, and will talk to you about supplementing feedings with formula or pumped breast milk.
Thanks to Janelle Aby MD, Stanford School Of Medicine, Newborn Nursery, and Lucille Packard Children's Hospital for the use of occasional photographs.