Most babies weigh between 12 and 15 pounds by 3 months old, though healthy weights vary quite a bit depending on birth weight, sex, and feeding method. During the first three months of life, babies gain roughly 1 ounce per day, which adds up to nearly 6 pounds over that stretch. A baby born at 7.5 pounds, for example, will typically be somewhere around 13 pounds by the 3-month mark.
Average Weight at 3 Months by Sex
Boys tend to be slightly heavier than girls at every stage of infancy. At 3 months, the 50th percentile (the statistical middle of all healthy babies) falls around 14.1 pounds for boys and 12.9 pounds for girls on the WHO growth charts, which are the standard the CDC recommends for all U.S. infants under 2 years old.
Those are midpoint numbers. A baby at the 25th percentile or the 75th percentile is equally healthy. What matters far more than any single number is whether your baby is following a consistent curve on the growth chart over time. A baby who has tracked along the 20th percentile since birth is growing exactly as expected, even though they weigh less than a baby on the 50th percentile.
How Babies Gain Weight in the First 3 Months
Newborns commonly lose 5 to 10 percent of their birth weight in the first few days as they adjust to feeding outside the womb. Most regain that weight by about two weeks old, and from there, the gains come quickly. Babies put on roughly 1 ounce (28 grams) per day through the first few months, making this the fastest period of weight gain they’ll experience.
Growth isn’t perfectly steady, though. Babies go through growth spurts, and 3 months is one of the classic timing points. During a spurt, your baby may seem hungrier than usual, feed more frequently, and act fussier for up to about three days. Sleep patterns can shift, too. These bursts are normal and tend to resolve on their own once the spurt passes.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often weigh about the same at 3 months, but their growth curves start to diverge shortly after. Formula-fed infants typically gain weight more quickly after the 3-month mark, while breastfed babies put on weight more slowly through the rest of the first year. This difference continues even after solid foods are introduced.
The CDC specifically notes that this slower weight gain in breastfed babies is normal and expected. It does not mean a breastfed baby is underfed. Both the WHO and CDC growth charts are built from data on breastfed infants, so a breastfed baby tracking along their percentile curve is right on target. Length growth is similar regardless of feeding method.
Signs Your Baby Is Getting Enough to Eat
Between weigh-ins at the pediatrician, diaper output is the most practical way to gauge whether your baby is well-fed. A 3-month-old should produce 6 to 8 wet diapers in a 24-hour period. Stool frequency varies more, especially among breastfed babies, who may go several days between bowel movements at this age without any cause for concern.
Other reassuring signs include steady alertness during wake windows, good skin color, and a baby who seems satisfied after feedings. By 3 months, most babies can support their own head when held upright, lift their head and chest while on their tummy, grab at toys, and begin cooing or making vowel sounds. These motor and communication milestones, alongside steady weight gain, paint a fuller picture of healthy development than weight alone.
When Weight Gain Falls Behind
Pediatricians look at a few specific red flags when evaluating an infant’s growth. A weight-for-age below the 5th percentile on standardized growth charts, a drop crossing two or more major percentile lines (for example, falling from the 50th to below the 10th), or weighing less than 80 percent of the expected weight for the baby’s length can all signal a growth concern sometimes called failure to thrive.
A single low reading at one visit isn’t necessarily alarming. Babies can weigh differently depending on when they last ate or had a diaper change. The pattern over multiple visits is what counts. If your baby’s weight is dipping across percentile lines rather than tracking a steady curve, your pediatrician will typically look at feeding patterns, check for underlying issues like reflux or food sensitivities, and may suggest more frequent weigh-ins to monitor the trend closely.
What the Growth Chart Actually Tells You
Growth charts plot your baby’s weight, length, and head circumference against thousands of other babies of the same age and sex. The result is a percentile ranking. A baby at the 40th percentile weighs more than 40 percent of babies that age and less than 60 percent. Neither high nor low percentiles are inherently better.
The WHO growth charts, updated and reviewed as recently as September 2024 by the CDC, are based on data from healthy children across six countries raised in conditions that support optimal growth. They describe how children grow when nutrition, healthcare, and environment are all favorable, making them a reliable benchmark. Your pediatrician plots your baby’s measurements at each well-child visit, and the resulting curve over time is far more informative than any single data point. A baby consistently tracking the 15th percentile is thriving. A baby who was at the 70th percentile and drops to the 30th percentile in a short window deserves a closer look, even though the 30th percentile is perfectly normal on its own.