The average 3-month-old weighs about 14 pounds (6.4 kg) for boys and 12.5 pounds (5.7 kg) for girls. These numbers come from the WHO growth charts that pediatricians use at well-child visits, and they represent the 50th percentile, meaning half of all babies weigh more and half weigh less. A healthy 3-month-old can fall anywhere from about 10 to 18 pounds depending on their birth weight, sex, and feeding pattern.
How Babies Reach This Weight
Newborns gain weight fast. In the first three months, most babies put on roughly 1 ounce (28 grams) per day. That pace slows around 4 months to about 20 grams per day. To put it in perspective, a baby who weighed 7.5 pounds at birth and gained an ounce a day would weigh close to 13 pounds by the end of month three, right in the normal range.
A common milestone pediatricians watch for: most healthy, full-term babies double their birth weight by 4 months. At 3 months, your baby is approaching but hasn’t quite hit that mark. A baby born at 7 pounds, for instance, would be expected to weigh somewhere around 12 to 13 pounds now, tracking toward 14 pounds by 4 months.
Boys vs. Girls
Boys tend to be slightly heavier than girls at every age during infancy. At 3 months, the difference is roughly 1 to 1.5 pounds on average. This gap is consistent and reflected in the separate growth charts your pediatrician uses for boys and girls. If you’re comparing your daughter to a friend’s son of the same age, that difference alone can explain a noticeable gap on the scale.
Breastfed vs. Formula-Fed Babies
Feeding method affects weight patterns more than most parents realize. Breastfed babies typically put on weight more slowly than formula-fed babies, and this difference becomes more apparent right around the 3-month mark. Formula-fed infants tend to gain weight more quickly after 3 months of age, and those differences in weight patterns continue even after solid foods are introduced later.
This means a breastfed baby who weighs a bit less than a formula-fed baby of the same age isn’t necessarily falling behind. The WHO growth charts, which are now the standard for children under 2, were designed based on breastfed infants and reflect this slightly slower gain as normal. If your pediatrician is using these charts (most do), a breastfed baby tracking along a lower percentile is still growing well as long as they’re following their own curve consistently.
What the Percentiles Actually Mean
Growth charts plot 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 completely normal. What matters far more than any single number is the trend over time. A baby who has been tracking along the 20th percentile since birth is doing exactly what’s expected. A baby who drops from the 60th to the 15th percentile over a couple of months is worth investigating, even though 15th percentile is technically “normal.”
Pediatricians look at weight, length, and head circumference together. A baby who is light but also short is proportionally small, which is different from a baby who is light but tall for their age. Your doctor plots all three at each visit to see the full picture.
Signs of Slow Weight Gain
Most babies gain weight just fine, but there are specific thresholds that signal a problem. Stanford Medicine Children’s Health identifies these red flags for full-term babies:
- Before 3 months: gaining less than about 1 ounce (30 grams) per day
- Between 3 and 6 months: gaining less than about two-thirds of an ounce (20 grams) per day
- Early on: not regaining birth weight by 10 to 14 days after birth
- At any point: a dramatic drop from the baby’s previous growth curve
If your baby is gaining less than these benchmarks, your pediatrician will likely want to evaluate feeding patterns, check for underlying issues, and possibly schedule more frequent weight checks. Occasional slower weeks are normal, especially during illness, but a sustained pattern of poor gain is something to address early.
Feeding at 3 Months
At this age, babies are still exclusively on breast milk or formula. For formula-fed babies, a general guideline is about 2.5 ounces of formula per day for every pound of body weight. So a 13-pound baby would need roughly 32 ounces per day, which is also the recommended upper limit for daily formula intake. Most 3-month-olds eat about 4 to 6 ounces per feeding, five to six times a day.
Breastfed babies are harder to measure by volume, but frequency is a good indicator. Most breastfed 3-month-olds nurse 7 to 9 times per day. Consistent wet diapers (six or more per day) and steady weight gain at checkups are the best signs that a breastfed baby is getting enough.
Practical Sizing at This Weight
If you’re wondering what this weight means for everyday purchases, most 3-month-olds fit into size 2 diapers, which are designed for babies between 12 and 18 pounds. Smaller 3-month-olds may still be in size 1 (8 to 14 pounds), while bigger babies could be moving into size 2 early. Clothing labeled “3 months” generally fits babies up to about 12.5 pounds, which means many average-sized 3-month-olds are already wearing clothes labeled “3 to 6 months” or “6 months.”