Most babies actually lose weight in the first week after birth. As long as yours is feeding well, making urine and stool, and doesn't drop more than 10% of birth weight, this isn't a problem. By two weeks, I expect a baby to return to birth weight. The average baby then gains about one ounce per day for the first month, and about one or two pounds a month until month six. Most babies double their birth weight by five or six months, and triple it by a year. Remember, though, these are averages. It's perfectly normal for an otherwise healthy baby to vary from these standards. I start to worry if a newborn loses more than 10% of birth weight, isn't making at least three or four wet diapers per day, doesn't seem satisfied after feedings, or doesn't get back to birth weight by two weeks.