At 3 months old, the average baby boy weighs about 14.1 pounds (6.4 kg) and the average baby girl weighs about 12.9 pounds (5.8 kg), based on World Health Organization growth standards. But “average” is just the middle of a wide, healthy range. A baby at the 15th percentile and a baby at the 85th percentile can both be growing perfectly well. What matters most is that your baby is following a consistent curve on their growth chart over time.
Typical Weight Ranges at 3 Months
The CDC recommends that healthcare providers in the United States use the WHO Child Growth Standard Charts for all children from birth to age 2. These charts are based on data from healthy breastfed infants across multiple countries, making them the current gold standard for tracking infant growth.
Here’s what the WHO charts show for 3-month-olds:
- Boys: Most fall between about 11.4 and 16.9 pounds (5.2 to 7.7 kg), with the 50th percentile at roughly 14.1 pounds.
- Girls: Most fall between about 10.6 and 15.4 pounds (4.8 to 7.0 kg), with the 50th percentile at roughly 12.9 pounds.
Percentiles describe where your baby falls compared to other babies of the same age and sex. A baby at the 25th percentile weighs more than 25% of babies and less than 75%. That’s not a problem. Pediatricians look for a baby to track along roughly the same percentile line from visit to visit. A baby who has been at the 20th percentile since birth and stays there is on a healthy trajectory. A baby who drops from the 60th percentile to the 15th percentile in a short period is the one who needs a closer look.
How Much Weight Babies Gain Per Month
Most full-term infants gain weight fastest in the first few months of life. During months one through four, expect roughly 1.5 to 2 pounds of weight gain per month. By 3 months, many babies have doubled or nearly doubled their birth weight, though this varies depending on birth size. After the 4-month mark, the rate of gain gradually slows as babies become more active.
A common growth spurt happens right around 3 months. These spurts typically last up to three days, during which your baby may seem hungrier than usual, fussier, and have disrupted sleep patterns. This is normal and temporary. You may feel like your baby suddenly can’t get enough milk or formula, but their appetite usually settles once the spurt passes.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to grow at similar rates in the first few months, but their patterns diverge after about 3 months. Healthy breastfed infants typically put on weight more slowly than formula-fed infants through the rest of the first year. Formula-fed babies gain weight more quickly after the 3-month mark, and this difference persists even after solid foods are introduced.
This doesn’t mean breastfed babies are underweight or underfed. The WHO growth charts were specifically designed around breastfed infant growth, so they reflect what healthy growth looks like for these babies. If your pediatrician is using these charts (as the CDC recommends), a breastfed baby tracking steadily along their own curve is doing exactly what’s expected. Length growth, meanwhile, is similar regardless of feeding method.
Weight Expectations for Premature Babies
If your baby was born early, the numbers above won’t apply the same way. Premature infants are tracked using “corrected age,” which adjusts for how early they arrived. A baby born 6 weeks premature who is now 3 months old would be evaluated as a 6-week-old on the growth chart. This adjusted-age approach is recommended until age 2.
For preterm infants weighing over about 4.4 pounds, a healthy weight gain target is roughly 20 to 30 grams per day (about 0.7 to 1 ounce). Babies born at very low birth weights often grow near or below the 5th percentile, but as long as their growth runs parallel to the normal curve, this is generally a healthy pattern. Catch-up growth does happen for many preemies, typically starting with head circumference and then followed by weight and length. Babies who were born small for their gestational age may catch up more slowly.
Signs Your Baby’s Weight Gain May Be Off Track
Pediatricians identify growth concerns by plotting your baby’s weight and length on standardized growth charts and watching for drops across percentile lines. At home, there are behavioral signs that can signal a problem before the next weigh-in:
- Excessive sleepiness: Sleeping more than expected for age, or falling asleep during feedings before finishing.
- Increased crying: More fussiness than usual without an obvious cause like a growth spurt.
- Fewer wet diapers: A noticeable drop in wet or dirty diapers can indicate insufficient intake.
- Lack of social engagement: Not mimicking facial expressions or interacting with people in age-appropriate ways.
Any one of these signs in isolation may not mean much. Babies have off days. But if you’re seeing a cluster of these behaviors, or if your baby is difficult to wake for feedings, that warrants a call to your pediatrician. Growth faltering (previously called “failure to thrive”) is identified through a pattern on the growth chart, not a single weigh-in, so the earlier concerns are raised the easier it is to address them.
Why the Number on the Scale Isn’t Everything
It’s tempting to compare your baby’s weight to other babies or to fixate on hitting the 50th percentile. But growth charts are population tools, not scorecards. Half of all healthy babies, by definition, fall below the 50th percentile. Your baby’s individual curve matters far more than any single number. A 3-month-old who weighs 11 pounds and has been tracking consistently along the 10th percentile since birth is in a very different situation than a 3-month-old who weighs 11 pounds after falling from the 50th percentile.
Genetics play a significant role too. Smaller parents tend to have smaller babies, and larger parents tend to have larger babies. Your pediatrician takes all of this into account when evaluating whether your baby’s growth is on track. If your baby is alert, feeding well, producing plenty of wet diapers, and meeting developmental milestones, their weight is likely right where it should be for them.