A four-month-old baby typically weighs between 12 and 16 pounds, though healthy weights span a wider range depending on sex, birth weight, and feeding method. The World Health Organization growth standards, which the CDC recommends for all children under two, place the median weight for a four-month-old girl at 6.4 kg (about 14.1 pounds) and slightly higher for boys.
Average Weight by Sex
Girls and boys follow slightly different growth curves from birth. For four-month-old girls, the WHO growth charts show these benchmarks:
- 5th percentile: 5.2 kg (about 11.5 pounds)
- 50th percentile (median): 6.4 kg (about 14.1 pounds)
- 95th percentile: 7.9 kg (about 17.4 pounds)
Boys at four months run roughly half a pound to a pound heavier at each percentile. A baby at the 15th percentile is just as healthy as one at the 85th, as long as they’re growing steadily along their own curve. The percentile itself matters far less than whether your baby is staying on a consistent trajectory over time.
What “Normal” Growth Looks Like at This Age
During the first few months of life, babies gain about one ounce (28 grams) per day. That pace starts to slow around the four-month mark to roughly 20 grams per day, or about one to one and a quarter pounds per month. This gradual deceleration is completely expected and not a sign that anything is wrong.
A common milestone parents hear about is doubling birth weight. Many babies reach this point somewhere between four and five months. If your baby was born at 7 pounds, you might expect them to be in the 13 to 15 pound range by now. But babies born smaller or larger will hit this milestone on their own timeline, and there’s no exact deadline.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t gain weight at the same pace, and the difference becomes noticeable right around four months. Breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies, in particular, tend to gain weight more quickly after about three months of age.
This doesn’t mean breastfed babies are underweight or malnourished. Their length growth is similar to formula-fed babies. The WHO growth charts were designed using data from breastfed infants as the standard, which is one reason the CDC recommends them for children under two. If your pediatrician is using older CDC-specific charts, a breastfed baby might look like they’re “falling behind” when they’re actually growing normally. It’s worth asking which chart your provider uses.
Breastfed babies at this age typically nurse 8 to 12 times in 24 hours. Some feedings will be long, others surprisingly short. Babies generally take what they need and stop when full, so variation between sessions is normal.
Premature Babies Need a Different Calculation
If your baby was born before 37 weeks, comparing their weight to the standard four-month benchmarks isn’t accurate. Pediatricians use something called corrected age, which subtracts the number of weeks your baby arrived early from their actual age. A baby born at 34 weeks (six weeks early) who is now four months old would be compared against the growth charts for a two-and-a-half-month-old.
Research supports using corrected age through at least 36 months for extremely and very preterm children. Without this adjustment, preterm babies are frequently misclassified as having growth problems when they’re actually on track. If your baby was premature and you’re worried about their weight, make sure the comparison is based on corrected age rather than calendar age.
When Weight Gain May Be Too Slow
Slow weight gain, sometimes called weight faltering, is identified by comparing your baby’s growth pattern to other children of the same age and sex. A single weigh-in that seems low isn’t particularly meaningful. What matters is the trend across multiple visits. A baby who was tracking along the 40th percentile and drops to the 10th over a couple of months is more concerning than a baby who has consistently been at the 10th percentile since birth.
Providers also look at whether weight is proportionate to length. A baby who is long and lean may simply have a slender build, while a baby whose weight is dropping relative to their length may not be getting enough calories. Signs you might notice at home include fewer than six wet diapers a day, persistent fussiness after feedings, or a baby who seems unusually sleepy and difficult to wake for feeds.
How Pediatricians Track Growth
Your baby’s weight at any single visit is just one data point. Pediatricians plot weight, length, and head circumference on growth charts at every well-child visit to build a picture over time. The pattern matters more than any individual number. Babies commonly shift percentiles during the first six months as they settle into their genetic growth trajectory, so a jump up or down by 10 to 15 percentile points isn’t automatically a red flag.
Weight-for-age charts show how your baby compares to peers, while weight-for-length charts show whether their weight is proportionate to their body size. Both pieces of information together give a much clearer picture than weight alone. If you’re curious about where your baby falls, you can ask your pediatrician to walk you through the chart at your next visit. Seeing the curve plotted across several months is often more reassuring than any single number on a scale.