The average 4-month-old weighs about 14.2 pounds (6.4 kg) for boys and 13.0 pounds (5.9 kg) for girls, based on the WHO growth standards used by pediatricians in the United States. Most healthy 4-month-olds fall somewhere between 12 and 17 pounds, and the normal range is wide. What matters more than hitting one specific number is whether your baby is growing steadily along their own curve.
Average Weight by Sex
Boys and girls follow slightly different growth tracks from birth. At 4 months, the 50th percentile (the statistical midpoint) for boys is about 14.2 pounds, while for girls it’s about 13.0 pounds. A baby at the 25th percentile is smaller than average but perfectly healthy, and the same is true at the 75th percentile on the heavier side. The full normal range spans roughly from the 5th to the 95th percentile:
- Boys: approximately 12.0 to 17.0 pounds
- Girls: approximately 11.0 to 15.8 pounds
Your pediatrician plots your baby’s weight on a growth chart at every well-child visit. The percentile your baby lands on at birth often sets the trajectory for the months ahead. A baby who starts at the 30th percentile and stays near it is growing exactly as expected, even though they weigh less than average.
How Fast Babies Gain Weight at This Age
In the first few months of life, babies gain roughly 1 ounce per day. Around 4 months, that pace naturally slows to about 20 grams (just under ¾ of an ounce) per day. Over the course of a month, that works out to gaining about 1 to 1¼ pounds.
A useful milestone to keep in mind: most babies double their birth weight by 4 to 5 months. So if your baby was born at 7 pounds, you’d expect them to be somewhere around 14 pounds by this point. Babies who were born larger or smaller will hit different absolute numbers but should still roughly double.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to weigh about the same during the first three months, but their growth patterns start to diverge right around the 4-month mark. Formula-fed infants typically gain weight more quickly after 3 months, while breastfed babies put on weight more slowly throughout the first year. This difference persists even after solid foods are introduced later on.
This doesn’t mean breastfed babies are underfed. The WHO growth charts, which are now the standard for children under 2 in the U.S., were built primarily from data on breastfed infants, so they reflect normal breastfed growth. If your pediatrician uses an older CDC chart designed from a mixed-feeding population, a breastfed baby might appear to “drop” in percentile when they’re actually growing normally. It’s worth asking which chart your doctor uses.
What the Percentile Number Actually Means
A percentile tells you how your baby compares to other babies of the same age and sex. If your baby is at the 40th percentile, 40% of babies weigh less and 60% weigh more. That’s it. Being at the 15th percentile doesn’t mean something is wrong, and being at the 90th percentile doesn’t mean your baby is overfed.
Pediatricians pay far more attention to the pattern over time than to any single measurement. A baby who has been tracking along the 20th percentile since birth is on a healthy trajectory. A baby who drops from the 60th percentile to the 15th percentile over two or three visits is showing a change in growth velocity, and that’s what prompts further evaluation.
When Weight Gain Is a Concern
The primary red flag isn’t a low number on the scale. It’s a sustained change in the speed of weight gain, where a baby steadily falls away from their expected growth curve. Clinicians sometimes call this “failure to thrive,” and it’s identified by difficulty attaining or maintaining appropriate weight over time, not by a single weigh-in.
Signs that your baby’s weight gain may need a closer look include consistently gaining less than about 3 to 4 ounces per week at this age, dropping across two or more percentile lines on the growth chart, or seeming unusually lethargic or unsatisfied after feeds. In most cases, the cause is straightforward, like a latch issue, insufficient milk supply, or a need to increase feeding frequency, and it’s very treatable once identified.
Getting an Accurate Weight at Home
If you’re tracking weight between doctor visits, accuracy matters. Weigh your baby in just a clean diaper, on the same scale each time, and at a similar time of day (before a feed is ideal). A baby who just finished a full bottle can weigh 4 to 8 ounces more than they did before the feed, which can skew your reading.
In clinical settings, nurses are trained to remove clothing down to a dry diaper, center the baby on the scale tray, and make sure nothing else is touching the baby or the scale. They accept two measurements that agree within 4 ounces of each other. At home, a digital infant scale gives the most reliable readings. Bathroom scales aren’t sensitive enough for the small increments that matter at this age.
Premature Babies and Corrected Age
If your baby was born before 37 weeks, their weight at 4 months of calendar age won’t line up with the standard charts. Pediatricians use “corrected age” instead, which adjusts for how early the baby arrived. You calculate it by subtracting the number of weeks of prematurity from your baby’s actual age. A baby born at 34 weeks (6 weeks early) who is now 4 months old has a corrected age of about 2.5 months, and their weight should be compared to 2.5-month norms.
This correction is used for growth tracking during the first two years. Most premature babies gradually catch up to their peers, but the timeline varies. Your pediatrician will let you know when your baby’s growth has converged enough to start using standard age on the charts.