Do not panic if your baby loses weight after delivery. Newborn babies normally lose 5 to 10% of their birth weight in a few days after delivery. This is because the baby loses fluid after birth. The baby will gain back all the weight after 10 to 14 days after they start breastfeeding.If, however, the baby has lost weight because of a sickness or premature birth, it will take up to 3 weeks to gain back the lost weight.Newborns gain about 30 g per day during the first month and grow about 1 to 1.5 inches in height during the same period.Newborns experience periods of rapid growth at 7 to 10 days and again at 3 and 6 weeks.ewborns gain about 30 g per day during the first month and grow about 1 to 1.5 inches in height during the same period. Newborns experience periods of rapid growth at 7 to 10 days and again at 3 and 6 weeks.