A typical 3-month-old boy weighs around 14 pounds (6.4 kg), and a typical 3-month-old girl weighs around 12.8 pounds (5.8 kg), based on the 50th percentile of the WHO growth charts. But “typical” covers a wide range. Babies anywhere from the 2nd to the 98th percentile are considered within normal limits, so a healthy 3-month-old might weigh anywhere from about 10 to 17 pounds depending on sex, birth weight, and genetics.
Average Weight by Sex
The WHO growth standards, which the AAP and CDC recommend for all children under 2, place the 50th percentile weights at 3 months as follows:
- Boys: approximately 14.1 pounds (6.4 kg)
- Girls: approximately 12.8 pounds (5.8 kg)
The 50th percentile is simply the midpoint, meaning half of babies weigh more and half weigh less. A baby at the 15th percentile is not underweight any more than a baby at the 85th percentile is overweight. In fact, there is no accepted clinical definition of “underweight” or “overweight” for children under 2. What matters far more than where your baby falls on the chart is whether they’re following a consistent curve over time.
Why the Trend Matters More Than the Number
Pediatricians pay less attention to a single weigh-in and more attention to the pattern across several visits. In the first few months, babies gain roughly 1 ounce (28 grams) per day. A baby who was born at the 30th percentile and stays near the 30th percentile at every checkup is growing exactly as expected, even if they weigh less than their peers.
The real red flag is a change in trajectory. If a baby drops steadily from one growth curve to a lower one over multiple visits, that shift in velocity is what prompts concern. This pattern, sometimes called failure to thrive, is identified by tracking weight over time rather than by any single measurement. A one-time dip after a stomach bug or a growth spurt that temporarily flattens the curve is normal. A sustained downward slide is not.
Birth Weight and the Doubling Milestone
Your baby’s birth weight is the single biggest predictor of what they’ll weigh at 3 months. A baby born at 6 pounds will naturally be lighter at 3 months than one born at 9 pounds, even if both are perfectly healthy. The general milestone to watch for is that most babies double their birth weight by around 6 months. At 3 months, your baby is roughly halfway to that milestone, so they should weigh noticeably more than at birth but not yet twice as much.
Premature babies follow a different timeline entirely. Their growth is typically tracked using an adjusted age (calculated from their due date, not their birth date), so a baby born four weeks early would be compared against the 2-month chart at their 3-month birthday.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies grow at slightly different rates, and both patterns are normal. Healthy breastfed infants typically put on weight more slowly than formula-fed infants over their first year, with the difference becoming more noticeable after about 3 months. Formula-fed babies tend to gain weight more quickly from that point forward.
This difference is one reason the WHO growth charts are preferred for all babies under 2. The WHO charts were built from data on predominantly breastfed infants, so they reflect how babies grow when fed according to current recommendations. Older CDC charts were based on a mix of feeding methods and tended to make breastfed babies look lighter than they should, sometimes triggering unnecessary worry.
How Feeding Amounts Relate to Weight
If you’re formula feeding, the general guideline is about 2.5 ounces of formula per day for every pound your baby weighs. So a 13-pound baby would need roughly 32 ounces spread across the day’s feedings. Breastfed babies are harder to measure precisely, but feeding on demand (typically 8 to 12 times in 24 hours at this age) usually provides the right amount.
Adequate intake shows up in predictable ways beyond the scale: six or more wet diapers a day, steady alertness during wake windows, and meeting motor milestones like holding their head up and batting at objects. If your baby is hitting those markers and gaining weight along a consistent curve, the exact number on the scale is less important than you might think.
What Percentiles Actually Mean
Growth percentiles rank your baby against 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 it. Percentiles are not grades. The AAP flags the 2nd and 98th percentiles as thresholds for further evaluation, not because being outside them is automatically a problem, but because it warrants a closer look at feeding, genetics, and overall health.
Genetics play an enormous role. Tall, large-framed parents tend to have babies who track in higher percentiles. Smaller parents tend to have babies who track lower. A baby consistently at the 10th percentile with parents who are both 5’3″ is following exactly the growth pattern you’d expect. Your pediatrician considers family size and build when interpreting the chart, which is another reason a single number in isolation tells you very little.