A 3-month-old boy typically weighs around 14.1 pounds (6.4 kg), and a 3-month-old girl typically weighs around 12.9 pounds (5.8 kg). Those are the 50th percentile values on the WHO growth charts, meaning half of healthy babies weigh more and half weigh less. But healthy 3-month-olds come in a wide range, and where your baby falls on the chart matters less than whether they’re growing steadily over time.
Average Weight by Sex
The World Health Organization growth standards, which the CDC recommends for all children under age 2, provide weight ranges based on large populations of healthy, breastfed infants worldwide. At 3 months, the typical ranges look like this:
- Boys: Most fall between about 11.4 and 16.5 pounds (5.2 to 7.5 kg). The 50th percentile is roughly 14.1 pounds (6.4 kg).
- Girls: Most fall between about 10.6 and 15.2 pounds (4.8 to 6.9 kg). The 50th percentile is roughly 12.9 pounds (5.8 kg).
These ranges cover roughly the 5th to 95th percentiles. A baby at the 15th percentile is just as healthy as one at the 85th, as long as they’re following a consistent curve. The number itself isn’t the goal. The trajectory is.
How Much Weight Babies Gain Per Week
In the first few months of life, babies gain about 1 ounce (28 grams) per day, which works out to roughly 7 ounces per week. That pace slows around 4 months to about 20 grams per day. So at 3 months, your baby is still in the faster growth phase, and you can generally expect noticeable weight changes between checkups.
A useful milestone to keep in mind: most babies double their birth weight by about 3.8 months (roughly 119 days). So if your baby was born at 7.5 pounds, you’d expect them to be approaching 15 pounds sometime around the 4-month mark. Boys tend to hit this milestone a bit earlier (around 111 days) and girls a bit later (around 129 days).
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t gain weight at the same rate, and this becomes more noticeable right around the 3-month mark. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to gain weight more quickly after about 3 months, and that difference persists even after solid foods are introduced later on.
Importantly, both groups grow in length at similar rates. The difference is primarily in weight. Formula-fed infants in one study doubled their birth weight by about 113 days, while breastfed infants took about 124 days. This doesn’t mean breastfed babies are underfed. The WHO growth charts were designed around breastfed infants specifically because their growth pattern is considered the biological standard. If your pediatrician is using the WHO charts (as recommended for children under 2), a breastfed baby tracking along a lower percentile is growing exactly as expected.
What Percentiles Actually Mean
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 completely normal. Pediatricians look for two things: whether the baby falls within a reasonable range (roughly the 5th to 95th percentiles) and whether they’re staying on or near their own curve over time.
What raises concern isn’t a low or high percentile by itself. It’s a sudden shift. If a baby has been tracking along the 60th percentile and drops to the 20th over a couple of visits, that change in trajectory is worth investigating, even though the 20th percentile is perfectly normal on its own. Similarly, a baby who has always tracked at the 10th percentile and continues to do so is growing well.
Why Birth Weight Matters at 3 Months
Your baby’s weight at 3 months is partly a reflection of where they started. Babies born at 6 pounds will naturally weigh less at 3 months than babies born at 9 pounds, even if both are gaining at a healthy rate. This is why pediatricians track growth as a curve from birth rather than comparing to a single target number.
It’s also worth noting that most newborns lose 5% to 10% of their birth weight in the first few days of life and regain it by about 10 to 14 days. The growth curve starts from there. So a baby who was 8 pounds at birth, dipped to 7.5 pounds in the first week, and is now 13.5 pounds at 3 months has been gaining steadily, even if that 13.5 number seems lower than average.
Growth Charts for Premature Babies
If your baby was born early, their weight at 3 months of calendar age won’t match the standard charts, and it’s not supposed to. Premature babies are assessed using “corrected age,” which subtracts the weeks of prematurity from their actual age. A baby born at 34 weeks (6 weeks early) who is now 3 months old has a corrected age of about 6 weeks, and their weight should be compared to the expected range for a 6-week-old.
This distinction is significant. Research shows that using calendar age instead of corrected age can dramatically overestimate growth problems. In one study, up to 89.8% of preterm infants were misclassified as underweight when assessed by calendar age alone. For very premature babies, corrected age should be used for all growth assessments through at least 36 months of corrected age. If your baby was premature and their doctor isn’t using corrected age, it’s worth bringing up.
Signs Your Baby Is Getting Enough Nutrition
Between weigh-ins, diaper output is the most practical way to gauge whether your baby is eating enough. From about day 5 onward, a well-fed infant produces at least 6 heavy wet diapers every 24 hours. In the early weeks, you should also see at least 2 soft yellow bowel movements per day, though stool frequency often decreases after the first month or two, especially in breastfed babies.
Other reassuring signs include your baby seeming satisfied after feeds, having good skin color and muscle tone, and being alert and active during awake periods. Babies who are consistently fussy after feeds, seem lethargic, or produce fewer wet diapers than expected may not be taking in enough milk, regardless of what the scale says. Weight checks at well-baby visits (typically scheduled at 1, 2, and 4 months) give your pediatrician the data points needed to confirm everything is on track.