One of the things that all new parents should know is that babies typically lose about 10 percent of their body weight by day three of life. (Breast-fed babies may lose a little more than bottle-fed ones because breastmilk doesn’t usually come in until the third day postpartum.)
Why do they lose that weight? Because babies are sleepy from the trauma of birth and feeding isn’t high on their list of priorities. What’s more, they’re also shedding some of the fluids they were born with, which can lighten their load. But don panic. By a week, they’ll have started gaining weight again, regardless they are breast-fed or formula-fed.