At 4 months old, the average baby weighs around 14 to 15 pounds and measures about 24 to 25 inches long, though healthy babies come in a wide range of sizes. What matters more than hitting a specific number is whether your baby is growing steadily along their own curve on a growth chart. Here’s what to expect and what to watch for.
Average Weight and Length at 4 Months
Boys tend to be slightly larger than girls at this age. On the WHO growth charts (the standard recommended by the CDC for all children under 2), the 50th percentile for a 4-month-old boy is roughly 15 pounds and 25 inches. For girls, it’s closer to 14 pounds and 24.5 inches. But “average” is just the midpoint. A baby at the 15th percentile and a baby at the 85th percentile can both be perfectly healthy. The key is consistency: a baby who has been tracking along the 25th percentile since birth is doing exactly what they should be doing, even if they’re smaller than their peers.
Between 4 and 6 months, babies typically gain about 1 to 1.25 pounds per month, according to Children’s Hospital of Philadelphia. That’s a noticeable slowdown from the rapid weight gain of the first three months, and it’s completely normal.
Breastfed vs. Formula-Fed Differences
If your baby is breastfed and seems smaller than a formula-fed baby the same age, that’s expected. Healthy breastfed infants typically put on weight more slowly than formula-fed infants, and this difference becomes more apparent after about 3 months. Formula-fed babies tend to gain weight faster from that point on, and the gap persists even after solid foods enter the picture. Length, on the other hand, grows at a similar rate regardless of feeding method.
This is one reason the WHO growth charts matter for babies under 2. They’re based on breastfed infants as the standard, so they won’t flag a healthy breastfed baby as underweight simply because they’re lighter than a formula-fed peer.
What Your Baby Should Be Doing Physically
Size isn’t the only measure of healthy growth at 4 months. Physical milestones reflect that your baby’s muscles and nervous system are developing on track. By this age, most babies can hold their head steady without support when you’re holding them, push up onto their elbows during tummy time, swing their arms at toys, bring their hands to their mouth, and hold a toy when you place it in their hand.
These milestones matter alongside weight and length. A baby who is growing steadily and hitting these physical markers is almost certainly doing well, regardless of where they fall on the percentile chart.
Diaper and Clothing Sizes
Most 4-month-olds fall into size 2 diapers (12 to 18 pounds) or size 3 diapers (16 to 28 pounds), depending on their build. A longer, leaner baby might stay in size 2 longer, while a heavier baby may already need size 3. For clothing, many 4-month-olds fit into 3-to-6-month sizes, though chubbier or longer babies often wear 6-to-9-month clothes already. Clothing labels vary wildly between brands, so go by your baby’s actual measurements rather than the age printed on the tag.
When Growth Is a Concern
Pediatricians watch for specific patterns rather than single measurements. A baby who drops across two or more percentile lines on the growth chart, or whose weight plateaus for several weeks, may need a closer look. At home, signs that something might be off include a baby who cries more than expected, sleeps excessively, falls asleep during feedings, or isn’t interacting socially the way you’d expect (like mimicking facial expressions). Trouble waking your baby is a more urgent sign that warrants a call to your pediatrician right away.
It’s also worth noting that a single weigh-in can be misleading. Babies have growth spurts and lulls. A pattern over several visits tells a much clearer story than any one number.
Premature Babies Need a Different Benchmark
If your baby was born early, their size at 4 months should be compared to their corrected age, not their actual age. You calculate corrected age by subtracting the number of weeks your baby was premature from their current age in weeks. A baby born at 32 weeks, for example, was 8 weeks early. At 4 months (16 weeks) after birth, their corrected age is only 2 months, and their size and development should be compared to a typical 2-month-old.
This adjustment is important because people may assume your baby is behind when they’re actually right on target for their corrected age. A 4-month-old with a corrected age of 2 months might just be starting to hold their head up and smile, which is exactly where they should be. Pediatricians use corrected age for growth and developmental assessments until at least 2 years old, sometimes longer for very premature babies.