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Most newborns will gain about 5-7 oz a week for the first few months. Many babies will have doubled their birth weight by about 3-4 months. At 4 months, weight gain will begin to look different for breastfed and formula fed babies. Research clearly shows that breastfed babies and formula fed babies grow at different rates starting at about 4 months of age.
My understanding is that the target 1oz (20g) weight gain per day is an average that is actually based upon bottle fed babies, because powered milk takes longer to digest it therefore also is in the babys system longer to convert into fat which is why a bottle fed baby gains weight quicker.