Most 3-month-old babies weigh between 11 and 16 pounds, with the average falling around 14.1 pounds for boys and 12.9 pounds for girls. That range is wide because healthy babies come in very different sizes, and your baby’s starting weight, sex, genetics, and feeding patterns all play a role in where they land.
Average Weight at 3 Months
The World Health Organization growth charts, which pediatricians use as the standard reference, place the 50th percentile for 3-month-old boys at roughly 14.1 pounds (6.4 kg) and for girls at about 12.9 pounds (5.8 kg). The 50th percentile simply means half of babies weigh more and half weigh less. It is not a target.
The normal range spans quite a bit. A boy at the 5th percentile might weigh around 11.4 pounds, while one at the 95th percentile could be over 17 pounds. For girls, the 5th percentile sits near 10.6 pounds and the 95th near 15.8 pounds. Any weight within that spread is considered normal as long as a baby is growing steadily along their own curve.
How Fast Babies Gain Weight 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 of age. From there, the growth rate is surprisingly fast. According to the Mayo Clinic, babies in the first few months gain about 1 ounce (28 grams) per day, which works out to roughly 5 to 7 ounces per week. At that pace, most babies have doubled their birth weight by the time they reach 3 to 4 months.
If your baby was born at an average weight of 7.5 pounds, doubling that puts them right around 15 pounds at 3 months, which lines up well with the growth chart averages. A baby born smaller, say 6 pounds, would be expected to weigh less at 3 months, and that’s perfectly fine as long as the growth curve is heading in the right direction.
What Affects Your Baby’s Weight
Several factors explain why 3-month-olds vary so much in size. Some are baked in from birth, while others relate to how a baby feeds and grows in those early weeks.
- Birth weight. Babies born before their due date tend to be smaller, while post-due-date babies often start larger. That head start (or deficit) carries through the first few months.
- Sex. Boys are typically heavier than girls at every age during infancy.
- Genetics. Your height and your partner’s height and build influence your baby’s frame. Taller, larger parents tend to have bigger babies.
- Birth order. First babies can be smaller than later siblings.
- Multiples. Twins and triplets often weigh less at birth and may stay on a lower growth curve in the early months.
Health conditions can also play a role. Babies with a cow’s milk allergy may have difficulty absorbing nutrients, and chronic conditions like congenital heart disease can increase calorie needs. In either case, weight gain may be slower than typical, and a pediatrician would track it closely.
Breastfed vs. Formula-Fed Babies
How you feed your baby can affect where they fall on the growth chart, especially starting around the 3-month mark. The CDC notes that healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year, with the difference becoming more noticeable after 3 months. Formula-fed babies tend to gain weight more quickly from that point forward.
This does not mean breastfed babies are underweight. The WHO growth charts were built primarily using data from breastfed infants, so a breastfed baby tracking along the 30th percentile is growing exactly as expected. Length growth is similar between breastfed and formula-fed babies, reinforcing that the difference is in fat accumulation, not overall development.
If your pediatrician is using older CDC growth charts instead of the WHO charts, a healthy breastfed baby can sometimes look like they’re lagging when they’re actually right on track. It’s worth asking which chart your doctor uses.
When Weight Gain Slows Down
Some parents notice their baby’s weight gain plateaus or dips around 2 to 3 months. A few common reasons explain this. Babies start sleeping longer stretches at night, which can mean fewer feedings in a 24-hour period. Illness in either the baby or a breastfeeding parent can temporarily reduce intake or milk supply. These slowdowns are usually brief and self-correcting.
What matters more than any single weigh-in is the pattern over time. Pediatricians look at whether a baby is following a consistent percentile curve rather than hitting a specific number. A baby who has been tracking along the 25th percentile since birth is thriving. A baby who drops from the 60th to the 15th percentile over a few weeks warrants a closer look, even if their actual weight still falls in the “normal” range.
Premature Babies and Corrected Age
If your baby was born early, the 3-month weight benchmarks above won’t apply directly. Pediatricians use corrected age, which adjusts for how early your baby arrived, until age 2. A baby born 6 weeks premature who is now 3 months old would be evaluated against growth standards for a 6-week-old, not a 3-month-old.
Once a premature baby reaches their original due date (term corrected age), they can be plotted on a standard growth chart using that corrected timeline. Growth rates for preemies after reaching term are similar to full-term babies, about 0.75 cm per week in length during the first three months. Many premature babies catch up to their full-term peers by 18 to 24 months, though the pace varies.