Try not to get hung up on weight gain. The basic expectations: A newborn will lose up to 10 percent of his body weight in the first week or so after birth, but then quickly gain it back. From there, pediatricians expect infants to double their birth weight by the 4-month checkup and triple it by their first birthday.
Take heart in the charts. The pediatrician will monitor your baby's growth on height and weight charts. Each chart should show an upward curve throughout the year with no sudden jump (say, from the 40th percentile to the 60th) at any one checkup.
But don't get obsessed with them. While height and weight percentiles are usually in the same ballpark, genetics plays a big role. A baby in the 90th percentile in height and only the 50th in weight won't surprise your pediatrician if Dad or you are also tall and lanky.