A 4-month-old boy typically weighs around 15.2 pounds (6.9 kg), and a 4-month-old girl typically weighs around 14 pounds (6.4 kg), based on the World Health Organization growth standards used by pediatricians. But healthy weights at this age span a wide range. What matters most isn’t hitting an exact number. It’s whether your baby is growing steadily along their own curve.
Average Weight Ranges by Sex
The WHO growth charts show that most 4-month-old boys fall between about 13 and 18 pounds, while most girls fall between about 12 and 16.5 pounds. These ranges cover the 5th through 95th percentiles, meaning the vast majority of healthy babies land somewhere in this window. A baby at the 10th percentile is just as healthy as one at the 90th, as long as they’re following a consistent growth pattern over time.
Percentiles describe where your baby falls compared to other babies of the same age and sex. If your baby is at the 25th percentile, that means 25% of babies weigh the same or less. It doesn’t mean your baby is underweight. Pediatricians watch the trajectory, not just the number. A baby who has tracked along the 15th percentile since birth is growing exactly as expected.
The Birth Weight Doubling Rule
A useful benchmark: most healthy, full-term babies double their birth weight by 4 months. So if your baby weighed 7 pounds at birth, you’d expect them to be around 14 pounds now. This is a rough guideline, not a strict cutoff. Babies who were born smaller or larger will naturally land at different points on the scale.
Growth rate slows down around this age. In the first few months, babies gain about 1 ounce per day. By 4 months, that drops to roughly 20 grams (about two-thirds of an ounce) per day. By 6 months, many babies gain 10 grams or less daily. So if your baby’s weight gain seems to be tapering compared to those early weeks, that’s normal.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often look different on the scale by 4 months. Formula-fed infants typically gain weight faster after about 3 months of age, while breastfed babies tend to put on weight more slowly throughout the first year. This difference continues even after solid foods are introduced. Both patterns are healthy.
This matters because growth charts can sometimes make breastfed babies look like they’re “falling behind” when they’re actually growing normally. The WHO growth charts, which most pediatricians now use for children under 2, were developed using primarily breastfed infants, making them a better reference for all feeding methods. If your breastfed baby is slightly lighter than a formula-fed baby the same age, that’s expected and not a reason to supplement.
Signs Your Baby Is Getting Enough
Between weigh-ins at the pediatrician’s office, a few everyday indicators can tell you whether your baby is eating enough. After the first week of life, babies who are feeding well produce at least six wet diapers per day. They should seem satisfied after feedings rather than fussy or constantly rooting, and they should be alert and active during awake periods.
At 4 months, formula-fed babies generally take in somewhere around 24 to 32 ounces of formula per day, spread across four to six feedings. Breastfed babies are harder to measure by volume, which is why diaper output and steady weight gain are the best indicators. If your baby is hitting those markers, the exact ounce count is less important.
When Weight Changes Are Worth Noting
Pediatricians don’t worry about a single weight measurement. Growth monitoring works by plotting a series of measurements over time, which reveals whether a baby is following a consistent pattern. What draws attention is a significant shift: a baby who was tracking along the 50th percentile and drops to the 10th over one or two visits, or one who jumps sharply upward.
Crossing one or two percentile lines can happen during normal growth spurts or brief illnesses. It becomes more meaningful when the shift is large or sustained. Measurements that fall below the 2.3rd percentile or above the 97.7th percentile are the clinical thresholds for evaluating whether something else might be going on. Even then, doctors weigh the numbers alongside your baby’s overall health, feeding patterns, and family history before drawing conclusions.
Premature babies follow a different timeline entirely. Their growth is typically tracked using a “corrected age,” which adjusts for how early they were born. A baby born two months early and now 4 months old would be compared against growth charts for a 2-month-old. If your baby was premature, the standard ranges above won’t apply directly.
What Actually Affects Your Baby’s Weight
Genetics play a bigger role than most parents realize. Tall, lean parents tend to have babies who track in lower weight percentiles. Shorter, stockier parents often see babies higher on the chart. Neither is better or worse. Your baby’s birth weight, sex, and feeding method all contribute to where they land, and none of these factors are something to “fix.”
Illness can temporarily slow weight gain. A cold, an ear infection, or a stomach bug might cause your baby to eat less for a few days. Most babies bounce back quickly and return to their usual growth curve within a week or two. A single slow week is not the same as a sustained pattern of poor growth.