Breastfed versus Formula Fed. Believe it or not but the milk you feed your baby will affect his weight gain. In general, babies who are breastfed gain less weight and gain slower than babies who are formula fed. A breastfed baby should gain between four and seven ounces every week during the first month of life. The first six months of life the baby should gain one to two pounds per month. Then, from six months to one year the average growth rate is one pound per month. Lengthwise, babies will grow about one inch per month until six months and then about one half inch per month until their first year.