A 3-month-old boy weighs about 14.1 pounds (6.4 kg) on average, while a 3-month-old girl weighs about 12.9 pounds (5.8 kg), based on World Health Organization growth standards. Most healthy 3-month-olds fall somewhere between 10 and 17 pounds, and the normal range is wide. What matters more than hitting an exact number is that your baby is gaining weight steadily along their own growth curve.
Average Weight by Sex
The WHO growth charts, which both the American Academy of Pediatrics and the CDC recommend for children under 2, show 50th percentile weights at 3 months of approximately 14.1 pounds for boys and 12.9 pounds for girls. But “average” is just the midpoint. A baby at the 15th percentile is just as healthy as one at the 85th, as long as they’re growing consistently.
Here’s a rough sense of the range at 3 months:
- Boys: 11.0 to 17.4 pounds (5th to 95th percentile)
- Girls: 10.2 to 16.0 pounds (5th to 95th percentile)
Your baby’s birth weight heavily influences where they land on this spectrum. A baby born at 6 pounds will likely still be lighter at 3 months than one born at 9 pounds, and that’s perfectly normal. Genetics, length, and feeding patterns all play a role too. Pediatricians look at growth as a trajectory, not a single snapshot.
How Fast Babies Gain Weight at This Age
In the first few months of life, babies typically gain about 1 ounce (28 grams) per day, or roughly 5 to 7 ounces per week. That pace is fastest in the early weeks and starts to slow down around the 3-month mark. Between 3 and 6 months, the expected gain drops to about two-thirds of an ounce per day (20 grams).
Most babies double their birth weight by around 4 to 5 months. So at 3 months, many babies are approaching (but haven’t quite reached) double their birth weight. If your baby was born at 7.5 pounds, a weight somewhere around 12 to 14 pounds at 3 months is right on track.
Breastfed vs. Formula-Fed Weight Patterns
Breastfed and formula-fed babies often weigh about the same at 3 months, but their growth patterns start to diverge after this point. Healthy breastfed babies typically put on weight more slowly than formula-fed babies through the first year. Formula-fed infants tend to gain weight more quickly after about 3 months, and that difference persists even after solid foods are introduced.
This is important because growth charts can make breastfed babies look like they’re falling behind when they’re actually growing exactly as expected. The WHO charts were designed using data from breastfed infants, which is one reason the AAP recommends them over older CDC charts for children under 2. If your pediatrician is using the right charts, a breastfed baby’s slightly slower gain after 3 months won’t be flagged as a concern. Length growth is similar regardless of feeding method.
Signs of Slow Weight Gain
A baby who is consistently gaining weight, even if they’re small, is generally doing fine. The concern arises when weight gain stalls or a baby drops significantly from their established growth curve. Stanford Medicine Children’s Health identifies several benchmarks to watch:
- Before 3 months: gaining less than about 1 ounce per day
- Between 3 and 6 months: gaining less than about two-thirds of an ounce per day
- After birth: not regaining birth weight by 10 to 14 days old
- At any age: a dramatic drop from the baby’s previous growth curve
That last point is the one pediatricians pay the most attention to. A baby who has been tracking along the 25th percentile and suddenly drops to the 5th percentile is more concerning than a baby who has always been at the 5th percentile. Crossing two or more major percentile lines in a short period usually prompts further evaluation. The AAP flags the 2nd and 98th percentiles as boundaries for identifying potentially suboptimal growth, but there is no formal definition of “underweight” for children under 2.
What Percentiles Actually Mean
Growth percentiles compare your baby to a large population of healthy babies the same age and sex. If your baby is at the 40th percentile for weight, that means 40% of babies weigh less and 60% weigh more. It does not mean your baby is getting a failing grade.
Pediatricians also track weight-for-length, which gives a sense of whether a baby’s weight is proportional to their body size. For babies under 6 months, BMI may actually be a better indicator of excess relative weight than weight-for-length. Research has shown that BMI at 2 months is a better predictor of obesity at age 2 than weight-for-length measurements are. Your pediatrician will track these ratios over time, so individual measurements at a single visit are less meaningful than the overall pattern across multiple checkups.
Why Your Baby’s Weight Might Differ
Plenty of factors push a 3-month-old’s weight above or below the median, and most of them are completely normal. Babies born to taller or larger parents tend to be bigger. Premature babies are typically plotted on growth charts using their adjusted age (age from their due date, not their birth date), so a baby born 4 weeks early would be compared to 2-month-old standards at their 3-month birthday.
Illness can temporarily slow weight gain. A cold, reflux, or a feeding difficulty might cause a brief plateau, followed by a catch-up period. Frequent feeding changes, like switching formulas or transitioning from breast to bottle, can also cause short-term fluctuations. If your baby is alert, producing plenty of wet diapers (at least 6 per day), and meeting developmental milestones, their weight is likely fine even if it doesn’t match the textbook average exactly.