Q. My 3-month-old son weighed 6 pounds, 9 ounces when he was born, and now weighs 18 pounds. Is it normal for babies to gain this much weight so soon?
A. The truth is, there is a wide range of what's considered normal when it comes to infant weight gain. Your son's weight is likely normal for him, though the average baby weighs about 13 pounds at this age and won't double his or her birthweight until 6 months. Weight gain is influenced by several factors, however.
If your infant receives primarily breastmilk, you don't have to worry about excess pounds since breastfed babies' body fat is metabolically different than that of bottle-fed infants and overfeeding is rarely a concern. No matter how you feed your son, though, be sure you're accurately interpreting his feeding cues; some infants cry because they want to be held and comforted rather than fed. Periodically, you can try introducing a bottle of water if you think your baby is getting too much formula. Most 3-month-old infants need approximately 25 ounces of formula a day, or between 2 and 2½ ounces per pound.
That said, overfeeding is not usually the cause of rapid weight gain in such a young baby. In most cases it has much more to do with the child's genetic body type. There are three basic body types—what we call "apples" (round all over), "pears" (round in the lower half of the body), and "bananas" (lean from head to toe). Apples and pears tend to retain weight, since they have a slower metabolism and burn fewer calories. Lean bananas have a high metabolism and burn more calories than apples and pears, even though all types may eat similar amounts. So your and your partner's body types play a significant role—if one or both of you is large, your baby may simply be following this genetic plan.
Since overfeeding at this age is rarely a problem, simply enjoy how your baby looks and feels now. Most large infants go through a slimming stage between 6 months and 9 months, when they begin playing, crawling and walking.