At 3 months old, the average baby boy weighs about 14.1 pounds (6.4 kg) and the average baby girl weighs about 12.9 pounds (5.8 kg), based on WHO growth standards. But “average” is just the midpoint of a wide healthy range. A 3-month-old boy anywhere from 11 to 17 pounds is typically growing normally, and for girls the range runs from about 10 to 16 pounds.
What matters more than hitting a specific number is whether your baby is following a consistent growth curve over time. Here’s what the ranges look like and what to pay attention to.
Average Weight by Sex at 3 Months
The WHO growth standards, which the CDC recommends for all U.S. infants from birth to age 2, break weight down by percentiles. A baby at the 50th percentile weighs more than half of babies the same age and sex, and less than the other half. Here’s what the range looks like at 3 months:
- Boys: 5th percentile is roughly 11 pounds (5.0 kg), 50th percentile is about 14.1 pounds (6.4 kg), and 95th percentile is around 17.2 pounds (7.8 kg).
- Girls: 5th percentile is roughly 10.2 pounds (4.6 kg), 50th percentile is about 12.9 pounds (5.8 kg), and 95th percentile is around 15.7 pounds (7.1 kg).
A baby at the 15th percentile is just as healthy as one at the 80th percentile. Percentiles describe size, not health. The concern starts when a baby’s weight falls below the 2nd percentile for their age and sex, or when they drop sharply from one percentile line to another over a short period.
How Fast Babies Gain Weight at This Age
During the first few months of life, babies gain about 1 ounce (28 grams) per day on average. That works out to roughly 5 to 7 ounces per week. By 3 months, this pace starts to slow slightly for many babies, which is completely normal and not a sign of a feeding problem.
A useful milestone to keep in mind: most babies double their birth weight by about 4 months. The average is 119 days, or just under 4 months. So if your baby was born at 7 pounds, you’d expect them to be approaching 14 pounds around the 3-month mark. Boys tend to hit this doubling point a bit sooner (around 111 days) than girls (around 129 days).
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies often weigh about the same at 3 months, but their growth curves start to diverge right around this age. Formula-fed infants typically gain weight more quickly after 3 months, while breastfed infants put on weight more slowly through the rest of the first year. This difference persists even after babies start eating solid foods.
Both patterns are normal. The WHO growth charts were designed around breastfed infants as the standard, so a breastfed baby who appears to “fall behind” on the chart compared to formula-fed peers is usually growing exactly as expected. Length growth is similar between the two groups regardless of feeding method. Formula-fed babies also tend to double their birth weight slightly earlier, at around 113 days compared to 124 days for breastfed babies.
When Weight Is a Concern
A single weigh-in doesn’t tell you much. Growth is a trend, and pediatricians look at the pattern across multiple visits. The two scenarios that raise a flag are a baby whose weight is consistently below the 2nd percentile for their age and sex, or a baby who drops across two or more major percentile lines over a few weeks. For example, a baby who was tracking at the 50th percentile and falls to the 10th by the next visit warrants a closer look.
On the other end, weight above the 98th percentile for the baby’s length can also prompt a conversation with your pediatrician, though in the first few months this is rarely a concern. Babies who seem chubby at 3 months often slim down once they become mobile.
Signs that your baby is getting enough nutrition include six or more wet diapers a day, steady alertness during awake periods, and meeting developmental milestones like holding their head up and tracking objects with their eyes. If your baby seems satisfied after feedings and is producing plenty of wet diapers, their weight is likely fine even if it doesn’t land near the 50th percentile.
Weight Expectations for Premature Babies
If your baby was born early, the numbers above won’t apply at 3 months of chronological age. Premature babies are measured using their “adjusted age,” which accounts for how early they arrived. To calculate it, subtract the number of weeks your baby was born early from their actual age in weeks. A baby born 6 weeks early who is now 3 months old has an adjusted age of about 6 weeks, so their weight should be compared to the chart values for a 6-week-old, not a 3-month-old.
This adjusted age is used for tracking growth and development until the child turns 2. Preemies often follow lower percentile curves initially and gradually catch up over the first one to two years. Their pediatrician will plot growth using the adjusted timeline, so the numbers at well-child visits may look different from what you’d expect based on calendar age alone.
What Matters More Than the Number
Your baby’s weight at 3 months is one data point. It’s useful, but it only becomes meaningful in context. Genetics play a significant role: smaller parents tend to have smaller babies, and that’s perfectly healthy. Birth weight sets the starting line, and a baby born at 6 pounds will likely weigh less at 3 months than one born at 9 pounds, even if both are growing at the same rate.
The growth curve matters more than any single measurement. A baby who has tracked steadily along the 20th percentile since birth is growing well. A baby who was at the 70th percentile and suddenly drops to the 20th needs evaluation. Consistency is the signal. Your pediatrician plots these points at every well-child visit specifically to catch shifts early, so keeping those appointments is the most practical thing you can do to stay on top of your baby’s growth.