Babies will lose about 7% of their body weight in the first week after birth but will steadily gain weight after that. If your baby is not gaining weight, their failure to thrive could indicate that they are not getting enough breast milk or that there is possibly an underlying health condition. Either way, weight gain is an important indicator that your baby is thriving, and if the gain is interrupted, then it is best to consult your doctor and a lactation specialist. This is usually a sound reason, to begin with, formula, but you need to find out the reason for your baby's weight loss or stagnation.