At 4 months old, the average baby boy weighs about 15.4 pounds (7 kg), and the average baby girl weighs about 14.1 pounds (6.4 kg). These numbers represent the 50th percentile on the World Health Organization growth charts, meaning half of all healthy babies weigh more and half weigh less. But a wide range is perfectly normal. A baby boy anywhere from about 12.6 to 18.3 pounds, or a baby girl from 11.4 to 17 pounds, falls within the typical 5th-to-95th percentile window.
What matters more than hitting a specific number is how your baby’s weight tracks over time on their own growth curve.
Typical Weight Gain in the First 4 Months
Newborns grow fast. In the first few months of life, babies gain roughly 1 ounce (28 grams) per day. That pace slows around 4 months to about 20 grams per day. Over the course of the first four months, most babies add somewhere between 4 and 7 pounds to their birth weight.
A common milestone parents hear about is doubling birth weight. Most babies reach that point by around 6 months old, not 4 months. So if your baby hasn’t doubled yet, that’s expected. A baby born at 7 pounds who weighs around 12 to 13 pounds at 4 months is right on track, even though they haven’t doubled.
Why the Growth Curve Matters More Than One Number
Pediatricians in the U.S. use WHO growth charts for children under 2. These charts show percentile lines that represent how your baby’s size compares to a large population of healthy children worldwide. Your baby’s percentile at a single visit is less important than the pattern those visits create over time.
A baby who has consistently tracked along the 20th percentile since birth is growing normally. That baby is smaller than average but following a steady, predictable curve. A baby who was at the 70th percentile at 2 months and drops to the 20th percentile at 4 months is a bigger concern, even though 20th percentile is technically “normal.” A downward crossing of two or more major percentile lines, or weight falling below the 3rd to 5th percentile, is what clinicians look for when evaluating whether a baby is getting enough nutrition. This pattern is sometimes called failure to thrive, and it signals that something may be interfering with feeding, absorption, or calorie needs.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies don’t grow at the same rate, and this catches some parents off guard. Healthy breastfed infants typically put on weight more slowly than formula-fed infants, with the difference becoming more noticeable after about 3 months. A breastfed baby who looks “smaller” than a formula-fed baby of the same age may be growing exactly as expected.
Length tends to be similar between the two groups. The WHO growth charts were developed primarily from data on breastfed infants, so they reflect normal breastfed growth patterns. If your pediatrician is using these charts (as recommended for babies under 2), a breastfed baby’s slower weight gain won’t be flagged unnecessarily.
Premature Babies Need an Adjusted Timeline
If your baby was born early, their weight at 4 months of calendar age won’t line up with the charts the same way. Pediatricians use “corrected age” for premature babies during the first two years of life. You calculate it by subtracting the number of weeks your baby arrived early from their actual age.
For example, a baby born at 32 weeks (8 weeks early) who is now 4 months old has a corrected age of just 2 months. Their weight, length, and developmental milestones should be compared to a 2-month-old, not a 4-month-old. This adjustment gives a much more accurate picture of whether growth is on track.
Signs of Healthy Growth Beyond the Scale
Weight is one data point. Plenty of other signals tell you your baby is growing well at 4 months. A well-nourished baby at this age typically:
- Produces enough wet diapers. Six or more wet diapers per day suggests adequate hydration and calorie intake.
- Holds their head steady without support when you hold them upright.
- Pushes up on forearms during tummy time.
- Swings at toys and brings hands to their mouth.
- Coos and makes vowel sounds like “oooo” and “aahh.”
- Smiles on their own to get your attention and chuckles when you play with them.
These milestones represent what 75% or more of babies can do by 4 months. A baby who is alert, feeding well, meeting these markers, and following their own growth curve is almost certainly getting what they need, regardless of whether their weight falls above or below the 50th percentile.
When Weight Gain Slows or Stalls
Some common reasons a 4-month-old might gain weight more slowly include a growth spurt that temporarily shifts their feeding patterns, a recent illness, or an undiagnosed feeding difficulty like a tongue tie or reflux. In breastfed babies, slow gain sometimes reflects low milk supply or inefficient latch, both of which are fixable with the right support.
A single weigh-in that looks low isn’t cause for alarm on its own. Pediatricians typically want to see a pattern across two or more visits before drawing conclusions. If your baby seems hungry after feedings, isn’t producing enough wet diapers, or seems unusually sleepy and hard to wake for feedings, those are more immediate signals worth raising at your next visit.