The average 3-month-old weighs between 12 and 14 pounds, though healthy babies can fall well outside that range. What matters more than hitting a specific number is whether your baby is gaining weight steadily and following a consistent curve on their growth chart.
Average Weight at 3 Months
Most 3-month-olds weigh somewhere around 12 to 14 pounds. Boys tend to be slightly heavier than girls at this age. But “average” is just the middle of a wide spectrum. A baby born at 6 pounds will look very different from one born at 9 pounds, and both can be perfectly healthy at 3 months.
A more useful benchmark than a single number is weight gain relative to birth weight. Babies typically double their birth weight by around 6 months and triple it by their first birthday. In the early months, the expected pace is roughly 1 ounce per day, or about 5 to 7 ounces per week. So by 3 months, most babies have gained somewhere around 5 to 6 pounds since birth.
Why the Growth Curve Matters More Than the Number
Pediatricians track your baby’s weight on a growth chart, which plots their measurements against thousands of other babies the same age. Your baby gets assigned a percentile. A baby in the 25th percentile isn’t “too small,” and a baby in the 75th percentile isn’t “too big.” These are simply positions on a normal distribution. A baby consistently tracking along the 15th percentile is growing just as well as one tracking along the 85th.
The concern isn’t where your baby falls on the chart. It’s whether they suddenly shift away from their established curve. When a baby’s weight drops across percentile lines and their length later drops too, that pattern can signal inadequate calorie intake. But some crossing of percentile lines is completely normal, especially in the first few months as babies settle into their own growth trajectory. There’s a real difference between normal variability and a pattern that needs attention, and your pediatrician is watching for exactly that at each well visit.
Growth Beyond the Scale
Weight is the easiest thing to measure, but it’s only one piece of the picture. At 3 months, babies also grow in length by about an inch per month on average, and their head circumference increases by roughly half an inch per month. Over the entire first year, babies grow about 10 inches longer than they were at birth.
Other signs your baby is growing well include consistent wet diapers (six or more per day by this age), steady alertness and increasing engagement with their surroundings, and hitting motor milestones like holding their head up and tracking objects with their eyes. A baby who is feeding well, producing plenty of wet diapers, and becoming more active is almost certainly getting enough nutrition, even if their weight seems low compared to a friend’s baby.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often grow at different rates, and that difference becomes noticeable right around the 3-month mark. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to gain weight more quickly after about 3 months of age.
This is why the WHO growth charts, which are based on breastfed babies raised in optimal conditions across six countries, are the recommended standard for all children under 2. If your breastfed baby is being plotted on a chart designed around formula-fed growth patterns, they might appear to be “falling behind” when they’re actually growing exactly as expected. The CDC recommends using the WHO charts for this age group for precisely this reason.
What Affects Your Baby’s Weight
Genetics plays a significant role. Smaller parents tend to have smaller babies, and larger parents tend to have larger ones. Birth weight also sets the starting line. A baby born premature or small for gestational age may still be catching up at 3 months, while a baby born large may slow their gain as they settle toward their genetic potential.
Feeding frequency and efficiency matter too. Some babies are vigorous eaters from the start. Others take longer to get the hang of breastfeeding or may deal with issues like tongue ties or reflux that make feeding less efficient. Illness can temporarily slow weight gain, as can a growth spurt where a baby stretches in length before filling out in weight. These fluctuations are a normal part of infant development, not automatic red flags.
Growth Spurts at 3 Months
Many babies go through a growth spurt right around 3 months. During a spurt, your baby may seem hungrier than usual, want to feed more frequently, be fussier, and sleep differently. These phases typically last a few days to a week. You might notice your baby suddenly outgrowing clothes or seeming longer overnight. Weight gain during and after a spurt can be uneven, with a big jump followed by a slower stretch, and that’s normal.
When Weight Is a Concern
A single weigh-in that seems high or low is rarely meaningful on its own. The pattern over time tells the real story. Signs that warrant a closer look include a baby who has dropped significantly from their growth curve over two or more visits, a baby who isn’t regaining weight after an illness, or a baby who consistently seems unsatisfied after feedings and isn’t producing enough wet diapers.
When weight does drop and length follows a few months later, that sequence specifically points to the baby not getting enough calories. But the reverse, where a baby is simply genetically smaller and has always tracked along a lower percentile, is not the same thing. Your pediatrician tracks these measurements at every well visit precisely to distinguish between the two.