The average 3-month-old weighs around 12 to 14 pounds, though healthy babies can fall above or below that range. Your baby’s individual growth pattern matters more than hitting one specific number, and pediatricians track that pattern using standardized growth charts at each well-child visit.
Average Weight at 3 Months
Based on the WHO growth standards (which the CDC recommends for all children under 2), a 3-month-old boy at the 50th percentile weighs roughly 14 pounds, while a 3-month-old girl at the 50th percentile weighs about 12.8 pounds. Boys tend to run slightly heavier than girls at every age during infancy.
The 50th percentile is just the midpoint. A baby at the 25th percentile is perfectly healthy, as is a baby at the 75th. What matters is that your baby’s weight follows a consistent curve over time rather than jumping dramatically up or dropping down. A baby who has tracked along the 20th percentile since birth is growing exactly as expected, even though they weigh less than “average.”
How Fast Babies Gain Weight at This Age
During the first few months of life, babies gain about 1 ounce per day, or roughly 2 pounds per month. That pace begins to slow around 4 months. So between your baby’s 2-month and 4-month checkups, you can generally expect a gain of about 4 pounds total.
A useful milestone to keep in mind: the typical baby doubles their birth weight by around 6 months. At 3 months, most babies are roughly halfway to that doubling point. If your baby was born at 7 pounds, a weight somewhere in the 10.5 to 14 pound range at 3 months would be on track.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies tend to grow at similar rates in the first couple of months, but their patterns 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 may appear to “slow down” compared to a formula-fed peer even though both are growing normally.
This is one reason the WHO growth charts are preferred for babies under 2. They were developed using data from breastfed infants and reflect a more natural growth trajectory. Older charts based primarily on formula-fed babies can make a healthy breastfed infant look like they’re falling behind when they’re not.
What Affects Your Baby’s Weight
Several factors influence where your baby falls on the growth chart at 3 months:
- Birth weight. Babies born larger tend to stay larger, and babies born smaller tend to stay smaller. Premature babies often track lower on standard charts for months.
- Genetics. Tall, lean parents are more likely to have long, lean babies. Shorter, stockier parents often have babies who are compact and heavier for their length.
- Sex. Boys are typically a pound or more heavier than girls at 3 months.
- Feeding frequency. Babies who eat more frequently or take in larger volumes per feeding naturally gain faster. Some babies are simply more efficient feeders than others.
Signs Your Baby Is Growing Well
Between weigh-ins at the pediatrician’s office, you can look for everyday signals that your baby is getting enough nutrition. At 3 months, a well-fed baby typically produces at least 5 to 6 wet diapers a day, seems satisfied after feedings rather than fussy and rooting, and is meeting developmental milestones like holding their head up and tracking objects with their eyes. Steady growth in length and head circumference also tells you things are on track, even if weight gain seems slow in a given week.
Short-term fluctuations are normal. A growth spurt at 3 months can cause a baby to feed ravenously for a few days, then level off. Illness can temporarily slow weight gain. One weigh-in that looks lower than expected is rarely cause for concern on its own. Pediatricians look for a pattern of two or more visits where weight crosses downward across percentile lines, which may signal a feeding issue worth investigating.
Practical Sizing at This Weight
If your 3-month-old weighs 12 to 14 pounds, they’re likely in Size 1 or Size 2 diapers. Size 1 fits 8 to 14 pounds, while Size 2 covers 12 to 18 pounds. Babies on the heavier end of the range, or those with chunky thighs, often do better sizing up even if their weight still technically fits the smaller size. Frequent blowouts or red marks on the legs are signs it’s time to move up.
For clothing, most babies at this weight are transitioning out of 0 to 3 month sizes and into 3 to 6 month sizes, though brands vary widely. Long, lean babies may need the next size up for length while the waist is still loose.