A 3-month-old baby typically weighs between 12 and 15 pounds, though healthy weights can range from about 10 to 18 pounds depending on sex, birth weight, and feeding method. Boys tend to be slightly heavier than girls at this age, with the average boy weighing around 14.1 pounds and the average girl around 12.9 pounds on standard growth charts.
Average Weight by Sex
The World Health Organization growth charts, which pediatricians in the U.S. use for children under 2, place the 50th percentile (the statistical middle) for a 3-month-old boy at roughly 14 pounds and for a girl at roughly 13 pounds. But “average” is just the midpoint of a wide healthy range. A baby at the 25th percentile is just as healthy as one at the 75th. What matters most is that your baby follows a consistent curve over time rather than hitting one specific number.
Here’s a rough guide to the range at 3 months:
- Boys: 11.4 lbs (10th percentile) to 16.3 lbs (90th percentile)
- Girls: 10.6 lbs (10th percentile) to 15.2 lbs (90th percentile)
How Much Weight Babies Gain in the First 3 Months
Newborns typically lose 5 to 10 percent of their birth weight in the first few days, then regain it by about two weeks old. After that, infants gain roughly 1 ounce per day during the first few months of life. That works out to about 1.5 to 2 pounds per month, so a baby born at 7.5 pounds might reasonably weigh around 13 pounds by the 3-month mark.
Growth isn’t perfectly linear, though. Some weeks your baby may gain more, other weeks less. A single weigh-in that looks low or high isn’t cause for concern on its own. Pediatricians look at the trend across multiple visits.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to grow at similar rates in the first couple of months, but their paths start to diverge right around the 3-month mark. Formula-fed infants typically gain weight more quickly after about 3 months of age, so a breastfed baby who seems lighter than a formula-fed peer may be growing perfectly normally.
The CDC recommends using the WHO growth charts for all children under 2, regardless of feeding method, because those charts are based on breastfed infants and reflect optimal growth patterns. If your pediatrician uses an older chart designed around formula-fed babies, a breastfed infant can appear to “fall behind” when they’re actually right on track.
What About Premature Babies
If your baby was born early, the number on the scale at 3 calendar months may look quite different from a full-term baby’s weight. That’s expected. Pediatricians use “corrected age” to evaluate preemie growth, meaning they subtract the weeks of prematurity from your baby’s actual age. A baby born 6 weeks early and now 3 months old would be assessed as a 6-week-old on the growth chart.
This corrected-age adjustment stays in place until age 2. Premature infants also have specialized growth charts for the period before they reach their original due date. Once they pass that milestone, standard WHO charts apply using the corrected age. So if your preemie looks small compared to other 3-month-olds, the comparison isn’t apples to apples.
Signs That Weight Gain May Be a Concern
Most babies who seem small or large are simply on their own healthy curve. But there are patterns that pediatricians watch for. A weight that falls below the 5th percentile for age, or that drops across two or more major percentile lines on the growth chart (for example, sliding from the 50th to the 10th), can signal a feeding or health issue that needs evaluation.
Day-to-day signs that your baby is getting enough to eat are often more reassuring than any single number. A well-fed 3-month-old typically produces at least 5 to 6 wet diapers a day, seems satisfied after feedings, and is alert and active during wake periods. Consistent weight loss, extreme fussiness during feeds, or very few wet diapers are more useful red flags than comparing your baby’s weight to a friend’s baby.
When Percentiles Shift
It’s common for babies to change percentile lines during the first few months as they settle into their genetically programmed growth pattern. A baby born large to smaller parents may drift from the 80th percentile down to the 40th, while a small newborn with tall parents may climb. These shifts are normal as long as they happen gradually and your baby stays on a steady curve afterward. Rapid or dramatic changes in either direction are what prompt further evaluation.