Most 4-month-old boys weigh around 14 to 15 pounds (6.4 to 6.8 kg), and most girls weigh around 12.5 to 14 pounds (5.7 to 6.4 kg). Those are the 50th percentile values on the WHO growth charts, which pediatricians in the United States use for all children under 2. But healthy babies come in a wide range of sizes, and your baby’s growth pattern over time matters far more than any single number on the scale.
Average Weight at 4 Months
The World Health Organization growth standards, recommended by both the CDC and the American Academy of Pediatrics for infants, show the following ranges for 4-month-olds:
- Boys: The middle 50% of boys fall between roughly 13.2 and 16.1 pounds (6.0 to 7.3 kg). The 50th percentile is about 14.3 pounds (6.5 kg).
- Girls: The middle 50% of girls fall between roughly 12.1 and 14.8 pounds (5.5 to 6.7 kg). The 50th percentile is about 13.2 pounds (6.0 kg).
A baby at the 10th percentile is just as healthy as one at the 90th percentile, as long as they’re growing consistently along their own curve. Percentiles describe where your baby falls compared to other babies of the same age and sex. They are not grades.
How Fast Babies Gain Weight at This Age
Weight gain slows down around the 4-month mark. In the first few months of life, babies typically gain about 1 ounce (28 grams) per day. By 4 months, that rate drops to about 20 grams per day, or roughly 4.5 ounces per week. This is completely normal and catches some parents off guard if they’ve gotten used to seeing rapid gains at earlier checkups.
A commonly cited milestone is that babies double their birth weight by around 6 months. So if your 4-month-old hasn’t doubled their birth weight yet, that’s expected. A baby born at 7 pounds, for example, would be on track to hit 14 pounds sometime between 4 and 6 months.
Breastfed vs. Formula-Fed Babies
Feeding method affects growth patterns starting right around this age. Healthy breastfed infants typically put on weight more slowly than formula-fed infants after about 3 months. Formula-fed babies tend to gain weight more quickly from this point forward, and those differences continue even after solid foods are introduced later on.
This doesn’t mean one feeding method produces healthier babies. The WHO growth charts were built primarily from data on breastfed infants, so a breastfed baby tracking along the 40th percentile is growing exactly as expected. A formula-fed baby may drift upward across percentile lines on those same charts, which can look like rapid growth but is a known pattern rather than a concern on its own. Length gain is similar between the two groups regardless of feeding method.
What Percentiles Actually Tell You
Your pediatrician plots your baby’s weight, length, and head circumference at each visit to track a growth curve over time. A single measurement is just a snapshot. What matters is whether your baby is following a consistent percentile range from one visit to the next. A baby who has always been at the 20th percentile and stays there is growing perfectly well.
Pediatricians look for patterns that suggest a problem: weight falling below the 2nd percentile for age and sex, weight-for-length rising above the 98th percentile, or a sharp drop that crosses two or more major percentile lines over a short period. These cutoffs come from WHO recommendations and signal that something may need a closer look, combined with the baby’s medical and family history. A small shift in percentile from one visit to the next is common and rarely meaningful on its own.
Premature Babies and Corrected Age
If your baby was born early, the numbers above won’t apply the same way. Growth expectations for premature infants are based on corrected age, which is your baby’s age minus the number of weeks they arrived early. A baby born 6 weeks premature who is now 4 months old would be evaluated as a 2.5-month-old for growth purposes. Pediatricians use corrected age for growth monitoring until age 2.
Babies born very early may also be tracked on specialized growth charts designed for premature infants during their first weeks and months, then transitioned to the standard WHO charts once they reach their original due date. If your baby was premature and you’re comparing their weight to the averages above, keep that age adjustment in mind.
What Healthy Growth Looks Like
At 4 months, you can generally tell your baby is growing well if they’re steadily outgrowing clothes, producing plenty of wet diapers (six or more per day), and seem satisfied after feedings. Babies at this age are also getting stronger, holding their heads up well and starting to reach for objects, which are signs that their overall development is on track alongside their weight gain.
The number on the scale at any single visit is less important than the trajectory. A 4-month-old who weighs 11 pounds but was 6 pounds at birth and has been climbing steadily is in a very different situation than one who weighed 9 pounds at birth and has stalled. Context is everything, and that’s exactly what growth charts are designed to capture over multiple visits.