Most 4-month-old boys weigh between 13 and 18 pounds, while most girls weigh between 12 and 17 pounds. The average sits around 15 pounds for boys and 14 pounds for girls, based on the World Health Organization growth charts pediatricians use for children under two. But a single number matters far less than your baby’s overall growth trend.
Why a Range Matters More Than One Number
Babies come in all sizes. A baby born at 6 pounds will look very different at four months than one born at 9 pounds, and both can be perfectly healthy. The key milestone pediatricians look for is whether your baby has roughly doubled their birth weight by 4 to 5 months of age. A baby born at 7 pounds, for instance, should be somewhere around 14 pounds by this point.
At this age, healthy infants typically gain about 1 to 1.25 pounds per month. That works out to roughly 4 to 5 ounces per week. Your pediatrician tracks this rate of gain over time on a growth chart, plotting each visit as a dot. What they’re looking for is a consistent curve, not a specific number. A baby who has always tracked along the 20th percentile is doing just as well as one cruising along the 80th.
What Pediatricians Actually Watch For
A single weigh-in can’t tell you much on its own. Growth concerns are identified by looking at multiple measurements over time. The red flag isn’t being small. It’s a pattern of falling away from the curve your baby has been following. If your baby was at the 50th percentile at two months and drops to the 15th percentile by four months, that downward shift is what prompts a closer look.
The American Academy of Pediatrics recently updated its guidelines and now uses a mathematical tool called a z-score to assess growth more precisely. Under these criteria, a child may be flagged for closer monitoring if their weight falls below roughly the 5th percentile for their length, if their rate of weight gain is significantly slower than expected, or if their weight drops substantially from where it had been tracking. The AAP also moved away from the older term “failure to thrive,” replacing it with “faltering weight” because the old label was imprecise and carried unnecessary stigma.
Breastfed vs. Formula-Fed Growth Patterns
If your baby is breastfed and seems lighter than a formula-fed baby the same age, that’s normal. Breastfed infants typically gain weight more slowly than formula-fed infants, and this difference becomes more noticeable after about 3 months of age. It continues even after solid foods are introduced later on. Both groups grow in length at similar rates, so the difference is specifically in weight gain speed.
This is one reason the WHO growth charts (which are based primarily on breastfed infants) are recommended for all children under two. Using charts based on formula-fed babies can make a healthy breastfed infant look like they’re falling behind when they’re growing exactly as expected.
How to Weigh Your Baby at Home
Your most accurate weights will always come from the pediatrician’s office, but if you want to track between visits, there are a few ways to get a reasonable reading at home.
Digital baby scales give the best results. Place the scale on a hard, flat surface like a kitchen or bathroom floor, not on carpet. Weigh your baby naked, ideally before a feeding and around the same time of day each time you measure. If your baby is squirming, wait for the number on the display to stabilize before recording it. Make sure their legs aren’t hanging off the edge of the scale, which throws off the reading.
If you don’t have a baby scale, you can use a workaround with your bathroom scale: weigh yourself alone, then weigh yourself holding your naked baby, and subtract. This method isn’t precise enough to detect small weekly changes, but it can give you a ballpark over the course of a few weeks. You can also try placing a baby bathtub on a digital bathroom scale, zeroing the scale with the tub on it, then setting your baby inside. Just make sure the tub doesn’t hang over the edges or touch the floor.
To check your scale’s accuracy, weigh something with a known weight, like a 1-kilogram bag of sugar. If the reading is close, your scale is reliable enough for home use.
Signs Your Baby Is Getting Enough
Weight is the most direct measure of whether your baby is eating enough, but it’s not the only one. At four months, a well-fed baby typically produces at least 6 wet diapers a day, is alert and active during awake periods, and is meeting movement milestones like holding their head steady and beginning to reach for objects. Skin that bounces back when gently pinched (rather than staying tented) is another simple sign of good hydration and nutrition.
If your baby seems content after feedings, is steadily outgrowing clothes, and your pediatrician isn’t concerned about the growth chart, the number on the scale is doing its job. The four-month well visit is one of the best opportunities to get a professional read on how your baby’s growth is tracking, since it comes with a fresh weight, length, and head circumference measurement all plotted on the same chart your doctor has been building since birth.