Most 4-month-old boys weigh between 13 and 18 pounds, with an average around 15 pounds. Girls at 4 months typically weigh between 12 and 16.5 pounds, averaging about 14 pounds. These ranges come from the WHO growth charts that pediatricians use at every well-child visit. But the number on the scale matters less than whether your baby is growing consistently along their own curve.
Average Weight by Sex
The World Health Organization growth standards show a wide range of healthy weights at 4 months. For boys, the 50th percentile (the midpoint) is roughly 15 pounds (6.8 kg). The 25th percentile is about 13.9 pounds, and the 75th percentile is about 16.3 pounds. For girls, the 50th percentile is approximately 13.7 pounds (6.2 kg), with the 25th percentile around 12.7 pounds and the 75th near 14.9 pounds.
A baby at the 25th percentile is not underweight. It simply means 25% of babies that age weigh less and 75% weigh more. Babies who were smaller at birth tend to stay in lower percentiles, and babies who were larger at birth tend to track higher. Both are perfectly normal as long as the trend stays relatively steady over time.
The Birth Weight Rule of Thumb
A quick way to check whether your baby’s growth is on track: most healthy, full-term newborns double their birth weight by 4 months and triple it by around their first birthday. So if your baby was born at 7.5 pounds, you’d expect them to be somewhere near 15 pounds at 4 months. This is a rough guideline, not a strict rule. Premature babies and those with medical conditions follow different timelines.
Why Growth Trends Matter More Than a Single Number
Pediatricians don’t focus on a single weigh-in. They look at how your baby’s weight plots over time on a growth curve. A baby who has been tracking along the 15th percentile since birth and stays there is growing well. A baby who drops from the 60th percentile to the 15th over two visits is a concern, even though both measurements fall within the “normal” range. The same goes for sudden jumps upward.
In the first few months of life, babies gain about 1 ounce per day. That pace slows around 4 months to roughly 20 grams (about two-thirds of an ounce) per day. By 6 months, many babies gain 10 grams or less per day. So if your baby’s weight gain seems to be tapering compared to the first couple of months, that’s expected.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t grow at the same rate, and that’s normal. Breastfed infants typically put on weight more slowly than formula-fed infants, especially after about 3 months. Formula-fed babies tend to gain weight more quickly during this period. These differences persist even after solid foods are introduced later on. Importantly, both groups grow in length at similar rates, so the difference is really about how quickly fat is deposited, not overall development.
If your breastfed baby is in a lower weight percentile than a formula-fed baby of the same age, that alone isn’t a problem. The WHO growth charts your pediatrician uses were developed primarily from breastfed infants, so they reflect breastfed growth as the standard.
How Much Your Baby Should Be Eating
At 4 months, formula-fed babies typically take about 6 ounces per feeding, spaced every 3 to 4 hours. That works out to roughly 24 to 32 ounces per day. Breastfed babies usually nurse every 2 to 4 hours, and the volume is harder to measure since it varies with each feeding. Either way, your baby should seem satisfied after most feedings and produce at least 6 wet diapers a day.
At 4 months, breast milk or formula should still be the only source of nutrition. Most pediatricians recommend waiting until closer to 6 months to introduce solid foods.
Signs of Poor Weight Gain
Slow weight gain, sometimes called failure to thrive, means a baby isn’t gaining at the expected rate. It’s not defined by a single low number on the scale but by a pattern of falling behind. Some signs that may point to a feeding or growth problem at this age include:
- Feeding difficulty: poor sucking, refusing the breast or bottle, or consistently short feedings
- Digestive issues: frequent vomiting or persistent diarrhea
- Low energy: sleeping much more than usual, a weak cry, or little interest in surroundings
- Muscle tone changes: feeling unusually stiff or floppy when you hold them
- Reduced engagement: not making eye contact during feedings, not moving around much, or seeming slow to reach developmental milestones
Any one of these on its own can be normal on a given day. But if several are happening consistently, or if your baby has dropped across two or more percentile lines on their growth chart, that warrants a closer look at feeding and overall health.
What Affects Your Baby’s Weight
Genetics play the biggest role. Tall, lean parents tend to have leaner babies. Shorter, stockier parents often have babies who sit higher on the weight chart. Birth weight itself sets the starting point: a baby born at 6 pounds and a baby born at 9 pounds will likely still be different sizes at 4 months, even if both are growing perfectly well.
Gestational age matters too. Babies born prematurely are often tracked using their “corrected age,” which is their age minus the number of weeks they arrived early. A baby born 6 weeks early and now 4 months old would be compared to growth standards for a 2.5-month-old. Your pediatrician will make this adjustment automatically.
Illness can temporarily slow weight gain. A baby recovering from a cold or stomach bug may plateau for a week or two and then catch up. This is usually not a long-term concern as long as the overall trend returns to their previous curve.