A typical 4-month-old weighs around 14 to 15 pounds, though healthy babies at this age can range from about 12 to 18 pounds depending on sex, birth weight, and feeding patterns. Boys tend to weigh slightly more than girls at this age. Rather than fixating on a single number, pediatricians look at whether your baby is growing consistently along their own curve on a growth chart.
Average Weight by Sex
Based on the CDC growth charts used in the United States, the 50th percentile (the statistical middle) for a 4-month-old boy is approximately 15.4 pounds, while for a 4-month-old girl it’s about 14.1 pounds. But “average” is just the midpoint of a wide range. A baby at the 25th percentile is just as healthy as one at the 75th percentile, as long as they’re following a steady growth pattern over time.
Here’s a rough breakdown of what different percentiles look like at 4 months:
- 10th percentile: about 12.5 lbs (girls) or 13.5 lbs (boys)
- 50th percentile: about 14.1 lbs (girls) or 15.4 lbs (boys)
- 90th percentile: about 16 lbs (girls) or 17.5 lbs (boys)
How Fast Babies Gain Weight at This Age
Between 4 and 6 months, most babies gain about 1 to 1.25 pounds per month. That’s noticeably slower than the rapid gains of the first three months, when many babies put on closer to 1.5 to 2 pounds monthly. This slowdown is normal and expected.
A common rule of thumb is that babies double their birth weight by around 6 months. So if your baby was born at 7.5 pounds, you’d expect them to be somewhere around 15 pounds by the half-year mark. At 4 months, most babies are well on their way to that milestone but haven’t reached it yet.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often weigh about the same in the first few months, but their growth paths start to diverge right around the 3- to 4-month mark. Formula-fed babies typically gain weight more quickly after 3 months, while breastfed babies tend to put on weight more slowly throughout the first year. These differences persist even after solid foods are introduced later on.
This matters because a breastfed baby who drops from the 50th to the 30th percentile between 3 and 6 months isn’t necessarily falling behind. Their growth pattern simply reflects the typical trajectory for breastfed infants. The CDC recommends using World Health Organization growth charts for children under 2, which are based on breastfed infants and better reflect that pattern.
What Matters More Than the Number
Pediatricians care less about where your baby falls on the chart and more about whether they’re staying on a consistent curve. A baby who’s been tracking along the 20th percentile since birth is growing normally. A baby who drops from the 60th percentile to the 15th over a couple of visits may need a closer look, even though the 15th percentile is technically within the normal range.
The American Academy of Pediatrics flags potential concern when a baby’s weight gain rate falls below the 2.3rd percentile for their age, or when their weight-for-length drops below the 5th percentile. These are the clinical thresholds that prompt further evaluation, not a single weigh-in that seems low. Your baby’s doctor is tracking this pattern across multiple visits, which is why keeping up with well-child checkups matters more than any single number on the scale.
Premature Babies Follow a Different Timeline
If your baby was born early, the weight ranges above won’t apply in the usual way. Growth for premature infants is tracked using “corrected age,” which is calculated from the original due date rather than the actual birth date. A baby born 6 weeks early who is now 4 months old would be assessed as a 2.5-month-old on a standard growth chart. This adjustment continues until age 2.
Preterm infants at 4 months corrected age typically gain between 15 and 25 grams per day, which works out to roughly 3.5 to 5.5 ounces daily. Their growth may look different from full-term peers for the entire first year, and that’s expected. Once a premature baby reaches their original due date equivalent, standard growth charts can be used alongside preterm-specific tracking.
Signs Your Baby Is Growing Well
Weight is one piece of the puzzle, but it’s not the only indicator of healthy growth at 4 months. Your baby should be producing at least 4 to 6 wet diapers a day, feeding regularly (whether breast or bottle), and showing steady developmental progress like improved head control and beginning to reach for objects. Most babies at this age are also getting noticeably longer and filling out through the cheeks, thighs, and belly.
If your baby seems satisfied after feedings, is alert and active during wake periods, and is steadily outgrowing clothes, those are all practical signs that growth is on track, regardless of where the number falls on a chart.