The average 3-month-old boy weighs about 14.3 pounds (6.4 kg), and the average 3-month-old girl weighs about 13.2 pounds (5.8 kg), based on the WHO growth standards the CDC recommends for all infants under 2. Most healthy 3-month-olds fall somewhere between 10 and 18 pounds, depending on sex, birth weight, and feeding patterns.
Average Weight by Sex
Boys and girls follow slightly different growth curves from birth. At 3 months, the 50th percentile (the statistical middle) looks like this:
- Boys: 14.3 pounds (6.4 kg)
- Girls: 13.2 pounds (5.8 kg)
These are midpoint numbers. A baby at the 25th percentile is just as healthy as one at the 75th percentile, as long as they’re gaining weight steadily along their own curve. A 3-month-old girl at the 25th percentile weighs around 11.7 pounds, while a boy at the 75th percentile is closer to 15.5 pounds. Both are completely normal.
How Much Weight Babies Gain at This Age
During the first few months, babies typically gain about 1 ounce (28 grams) per day, or roughly 5 to 7 ounces per week. That pace means most infants have doubled their birth weight by around 4 to 5 months. If your baby was born at 7.5 pounds, you’d expect them to be somewhere around 13 to 15 pounds by the 3-month mark.
Weight gain isn’t perfectly linear, though. Some weeks your baby might gain a full pound, and other weeks barely half that. What matters is the overall trend over several weeks, not any single weigh-in. Pediatricians track this at well-baby visits by plotting your baby’s weight on a growth chart and watching whether the curve stays roughly in the same percentile range over time.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to grow at similar rates for the first three months, but their patterns start to diverge right around this age. Healthy breastfed infants typically put on weight more slowly than formula-fed infants after 3 months. Formula-fed babies gain weight more quickly in the second half of the first year.
This means a breastfed baby who was tracking at the 50th percentile might gradually drift toward the 40th or 35th percentile compared to formula-fed peers, and that’s perfectly normal. The WHO growth standards were built primarily from data on breastfed infants, so they reflect this pattern. If your pediatrician is using the WHO charts (as the CDC recommends for children under 2), a breastfed baby’s slower gain after 3 months won’t be misread as a problem.
What Percentiles Actually Mean
A percentile tells you how your baby compares to other babies of the same age and sex. If your 3-month-old is at the 30th percentile, that means 30% of babies weigh less and 70% weigh more. It does not mean your baby is underweight. Babies who are consistently at the 10th percentile are usually just smaller, not unhealthy.
The concern isn’t where your baby sits on the chart. It’s whether they’re crossing percentile lines rapidly in either direction. A baby who drops from the 60th percentile to the 15th over a couple of months is worth investigating, not because the 15th percentile is dangerous, but because the steep downward shift could signal a feeding issue or an underlying condition. Identifying slow weight gain requires valid weight measurements over time, not a single data point. That’s why consistent well-baby visits matter more than any individual number on the scale.
Premature Babies Need an Adjusted Timeline
If your baby was born early, their expected weight at 3 months of calendar age is different from a full-term baby’s. Pediatricians use “corrected age” to account for this. You calculate it by subtracting the number of weeks your baby was born early from their actual age.
For example, a baby born at 34 weeks (6 weeks early) who is now 3 months old has a corrected age of about 6 weeks. Their weight should be compared to the growth chart values for a 6-week-old, not a 3-month-old. This adjustment is typically used until age 2, and it makes a significant difference in the early months. A preemie who looks small compared to full-term 3-month-olds may be growing perfectly on track for their corrected age.
Signs Your Baby Is Gaining Well
Between pediatrician visits, you probably don’t have a precise infant scale at home. A few everyday indicators can tell you whether your baby is getting enough nutrition. After the newborn stage, expect at least 6 wet diapers per day. Fewer than that can signal dehydration or inadequate intake. You should also see regular bowel movements, though the frequency varies widely (some breastfed babies go several days between stools, and that’s normal after the first month).
Other signs of healthy growth include your baby being alert and active during wake periods, steadily outgrowing clothes and diapers, and having good muscle tone. If your baby seems satisfied after feedings, is meeting developmental milestones, and their skin springs back when gently pinched, those are all reassuring signals that weight gain is on track, even if you don’t know the exact number.
Why Birth Weight Matters
Your baby’s birth weight sets the starting point for their entire growth curve. A baby born at 6 pounds will likely weigh less at 3 months than one born at 9 pounds, even if both are gaining at a perfectly healthy rate. Genetics play a large role here. Tall parents tend to have longer, heavier babies, and smaller parents often have smaller babies who track along lower percentiles throughout infancy.
It’s also common for babies to shift percentiles in the first few weeks as they settle into their genetically programmed growth pattern. A baby born large because of gestational diabetes, for example, might gradually move down to a lower percentile that better reflects their genetic size. Similarly, a baby born small might climb upward. These early shifts are expected. By 3 months, most babies have found the percentile range they’ll follow for the rest of their first year.