Most 3-month-old babies weigh between 11 and 17 pounds, depending on sex and birth weight. Boys tend to run slightly heavier than girls at this age. The average 3-month-old boy weighs about 14.3 pounds, while the average girl weighs around 13.2 pounds. But “average” is just the middle of a wide, healthy range, and your baby’s individual growth pattern matters more than any single number on the scale.
What the Growth Charts Show
Pediatricians in the United States use the World Health Organization (WHO) growth charts for all children under 2. These charts were built from data on healthy, breastfed infants raised in optimal conditions across multiple countries, so they represent how babies are designed to grow rather than just how they happen to grow in one population. The CDC recommends them over its own older charts for this age group.
On the WHO chart, your baby’s weight is plotted as a percentile. A baby at the 25th percentile weighs more than 25% of babies the same age and sex. A baby at the 75th percentile weighs more than 75%. Both are perfectly normal. What doctors pay attention to is whether your baby stays on roughly the same curve over time, not which curve they’re on.
Typical Weight Ranges at 3 Months
Here’s what the 5th through 95th percentiles look like at 3 months, which covers the vast majority of healthy babies:
- Boys: approximately 11.6 pounds (5th percentile) to 17.0 pounds (95th percentile), with 14.3 pounds at the 50th percentile
- Girls: approximately 10.6 pounds (5th percentile) to 15.8 pounds (95th percentile), with 13.2 pounds at the 50th percentile
Birth weight plays a big role. A baby born at 6 pounds will naturally be lighter at 3 months than one born at 9 pounds, even if both are growing at the exact same healthy rate. Between 1 and 3 months of age, babies typically gain about 1.5 to 2 pounds per month. That pace is fast compared to older infants. Most babies double their birth weight by around 6 months.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies follow slightly different growth curves, and this is normal. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. The difference becomes more noticeable after about 3 months, when formula-fed babies tend to gain weight more quickly. This gap continues even after solid foods are introduced.
If your breastfed baby is tracking along the lower percentiles but following a consistent curve, that’s not a problem. The WHO charts were specifically designed around breastfed infants, so a breastfed baby tracking at the 20th percentile is growing exactly as expected for their pattern. Trouble shows up when a baby drops significantly from one percentile curve to a much lower one over successive visits.
Growth Spurts Around 3 Months
Three months is one of the common ages for a growth spurt, and it can temporarily change your baby’s eating and sleeping habits in ways that feel alarming. During a spurt, you might notice your baby suddenly wanting to feed much more often (or occasionally less), sleeping more or less than usual, and being fussier than normal. These bursts are short, typically lasting up to about three days, and your baby’s appetite and mood usually settle back down afterward.
A growth spurt can also make it look like your baby’s weight jumped between checkups. That’s expected. Your pediatrician is looking at the overall trend across months, not one weigh-in.
When Weight Gain Raises Concerns
There’s no single cutoff that defines “too light” or “too heavy” at 3 months. Doctors look for patterns, not snapshots. The main red flag is a baby who is steadily falling off their expected weight curve on the growth chart over multiple visits. A baby who was tracking along the 40th percentile and drops to the 10th over a couple of months warrants a closer look, while a baby who has consistently been at the 10th percentile since birth is likely just a smaller baby.
Signs that a baby isn’t getting enough nutrition can include fewer than six wet diapers a day after the first week, persistent lethargy, and a lack of interest in feeding. On the other end, very rapid weight gain is less commonly flagged at this age, since fast growth in the first few months is biologically normal.
What Matters More Than the Number
Your baby’s weight at a single visit tells you less than you might think. The real information comes from watching how that weight changes over time. A baby who is alert, feeding well, producing plenty of wet and dirty diapers, and meeting developmental milestones is almost certainly growing fine, regardless of where they land on the percentile chart. Some families are built small, some are built big, and babies inherit those blueprints from day one.
If you want to track growth between pediatric visits, weigh your baby on the same scale each time, ideally without clothes or a diaper, and at roughly the same time of day. Small differences from one home weigh-in to the next are meaningless. The pattern over weeks and months is what counts.