The average 3-month-old boy weighs about 14.1 pounds (6.4 kg), and the average 3-month-old girl weighs about 12.9 pounds (5.8 kg), based on the World Health Organization growth standards used by pediatricians in the U.S. But “average” is just the midpoint on a wide spectrum. A healthy 3-month-old can weigh anywhere from about 10 to 18 pounds depending on sex, birth weight, genetics, and feeding method. What matters most isn’t a single number on the scale but whether your baby is gaining weight steadily along their own growth curve.
Average Weight by Sex
The WHO growth charts, which the CDC recommends for children under 2, place the 50th percentile for 3-month-old boys at roughly 14.1 pounds and for girls at roughly 12.9 pounds. The 50th percentile means half of healthy babies weigh more and half weigh less. Here’s what the broader range looks like:
- Boys: The 5th percentile is about 11.0 pounds, the 50th is about 14.1 pounds, and the 95th is about 17.6 pounds.
- Girls: The 5th percentile is about 10.2 pounds, the 50th is about 12.9 pounds, and the 95th is about 16.4 pounds.
A baby at the 15th percentile is just as healthy as a baby at the 85th percentile, as long as they’re growing consistently. Pediatricians look at the trajectory over multiple visits, not a single weigh-in.
How Fast Babies Gain Weight
During the first few months of life, babies gain about 1 ounce (28 grams) per day, according to the Mayo Clinic. That works out to roughly 5 to 7 ounces per week. This pace slows around 4 months to about 20 grams per day.
For infants from birth to 3 months specifically, the expected median weight gain is 26 to 31 grams per day. Most babies roughly double their birth weight by 4 to 5 months. So if your baby was born at 7.5 pounds, you’d expect them to be somewhere around 12 to 14 pounds at the 3-month mark, trending toward 15 pounds by 4 to 5 months.
Growth Curves Matter More Than a Single Number
Pediatricians don’t evaluate your baby’s weight by comparing it to a fixed target. They plot it on a growth chart at each visit and watch the pattern over time. A baby who has been tracking along the 25th percentile since birth and continues to do so is growing perfectly well. A baby who drops from the 60th percentile to the 15th percentile over two visits is more concerning, even if 15th percentile is technically “normal.”
This is why the AAP’s well-child visit schedule includes appointments at birth (3 to 5 days old), 1 month, 2 months, and 4 months. Each visit includes a weight check that adds another data point to your baby’s curve. If you’re worried between scheduled visits, most pediatric offices are happy to do a quick weight check without a full appointment.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies don’t follow the same weight trajectory. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year, and the difference becomes more noticeable after about 3 months. Formula-fed babies tend to gain weight more quickly from that point onward, even after solid foods enter the picture.
Importantly, both groups grow in length at similar rates. The difference is primarily in weight gain speed. This means a breastfed baby sitting at the 30th percentile for weight at 3 months isn’t falling behind. The WHO growth charts your pediatrician uses were built from data on breastfed infants, so they already account for this pattern. If your baby’s doctor is using an older CDC chart designed primarily from formula-fed data, a breastfed baby’s slower gain might look more dramatic than it actually is.
When Weight Gain Is Too Slow
There’s no single cutoff that defines inadequate weight gain. Doctors look for a pattern of steadily falling off the expected growth curve over time rather than a one-time low reading. A baby who was sick for a week and lost a little weight will usually bounce right back. A baby whose weight curve is flattening or dropping across multiple visits may need further evaluation.
Signs at home that your baby is getting enough to eat include six or more wet diapers a day, regular bowel movements, alertness during waking hours, and steady growth in length and head circumference alongside weight. If your baby seems consistently hungry after feeding, is unusually sleepy, or isn’t producing enough wet diapers, it’s worth checking in with your pediatrician sooner rather than waiting for the next scheduled visit.
Adjustments for Premature Babies
If your baby was born early, weight expectations are different. Pediatricians use “corrected age” rather than actual age when assessing growth. Corrected age is calculated by subtracting the number of weeks your baby was born early from their actual age. So a baby born 6 weeks premature who is now 3 months old would be evaluated as a 6-week-old on the growth chart.
Premature infants have their own specialized growth charts for the period before they reach their original due date. Once they hit term-equivalent age, doctors typically switch to the standard WHO charts but continue using corrected age until the child is 2 years old. This adjustment gives preemies a fair comparison and prevents unnecessary worry over weight that’s perfectly appropriate for their developmental stage.
What Influences Your Baby’s Weight
Genetics plays a larger role than many parents realize. Tall, large-framed parents tend to have bigger babies, and smaller parents tend to have smaller ones. Birth weight also sets the starting point: a baby born at 6 pounds is unlikely to weigh the same at 3 months as a baby born at 9 pounds, even if both are perfectly healthy.
Feeding frequency and efficiency matter too. Some babies are more efficient feeders and extract more calories per session. Illness, reflux, and food sensitivities can temporarily slow weight gain. And normal growth isn’t always smooth. Babies sometimes plateau for a week or two and then have a growth spurt. The overall trend across weeks and months is what counts.