The average 3-month-old weighs about 14 pounds (6.4 kg) for boys and 13 pounds (5.8 kg) for girls. That said, healthy babies come in a wide range of sizes, and where your baby falls on the growth chart matters less than whether they’re gaining weight consistently over time.
Average Weight at 3 Months
According to the World Health Organization growth standards, a 3-month-old boy at the 50th percentile weighs roughly 14.3 pounds (6.4 kg). Girls tend to be slightly lighter, averaging about 13 pounds (5.8 kg) at the same age. But the normal range is broad. A boy at the 5th percentile weighs around 11.5 pounds (5.2 kg), while one at the 95th percentile weighs about 17 pounds (7.7 kg). Both are perfectly healthy.
Pediatricians use these percentile charts not to judge any single weigh-in but to track a baby’s growth curve over multiple visits. A baby who consistently tracks along the 15th percentile is growing normally, even though they weigh less than most babies their age. What raises concern is a sharp jump between percentile lines, either up or down, over a short period.
How Fast Babies Gain Weight
During the first three months, babies gain about 1.5 to 2 pounds per month, or roughly 1 ounce per day. That’s the fastest growth rate most humans ever experience. By 3 months, many babies have doubled or nearly doubled their birth weight. A baby born at 7.5 pounds, for instance, might weigh 13 to 15 pounds by the 3-month mark.
It’s common for newborns to lose 5 to 10 percent of their birth weight in the first few days after delivery. Most regain that weight within 10 to 14 days. From that point forward, the daily gains begin adding up quickly. If your baby was slow to regain birth weight but has been gaining steadily since, their 3-month weight may sit a bit lower than average without being a concern.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to grow at similar rates during the first two to three months, but their trajectories start to split after that. Breastfed infants typically put on weight more slowly than formula-fed infants through the rest of the first year. Formula-fed babies, by contrast, tend to gain weight more quickly after about 3 months of age.
This difference is normal and well-documented. The CDC notes that pediatricians should use the WHO growth charts (which are based primarily on breastfed infants) for all children under 2, regardless of feeding method. If your breastfed baby seems to slow down compared to a formula-fed baby of the same age, that’s expected biology, not a sign of underfeeding.
What Affects Your Baby’s Weight
Several factors shape where a baby falls on the growth chart at 3 months:
- Birth weight. Bigger newborns tend to be bigger 3-month-olds. Genetics play a large role here, including the size of both parents.
- Sex. Boys are heavier than girls on average at every age during infancy.
- Feeding frequency and intake. Babies who eat more frequently or take in larger volumes at each feeding gain faster, though there’s a wide range of normal feeding patterns.
- Gestational age. Babies born prematurely are expected to weigh less at 3 calendar months than full-term babies. Pediatricians use “corrected age” for preemies, meaning they subtract the weeks of prematurity from the 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 approach is standard until the child turns 2.
Signs of Healthy Growth
Weight is one piece of the picture, but it’s not the only one. A baby who is growing well at 3 months typically shows several other signs: producing at least 6 wet diapers a day, feeding 8 to 12 times in 24 hours (or 6 to 8 times for formula-fed babies), seeming satisfied after feedings, and becoming more alert and active between meals.
Length and head circumference also get tracked alongside weight. A baby whose weight drops but whose length keeps climbing may simply be stretching out. Pediatricians look at all three measurements together and compare them to previous visits to spot any trends worth investigating.
When Weight Gain Is Too Slow or Too Fast
A baby who gains less than about 4 ounces per week during the first three months, or who drops two or more percentile lines on the growth chart, may need a closer look. Slow weight gain can result from feeding difficulties, low milk supply, tongue ties that make latching hard, or an underlying health issue. In most cases, adjusting feeding frequency or technique resolves the problem.
On the other end, unusually rapid weight gain in the first few months is less commonly flagged as a concern, since young infants regulate their intake fairly well. Overfeeding is more of a risk with bottle-fed babies who may drink past fullness. Watching for hunger cues rather than encouraging a baby to finish every bottle helps keep intake in a healthy range.