A typical 3-month-old weighs around 12 to 14 pounds and measures about 23 to 24 inches long, though healthy babies vary quite a bit in both directions. By this age, most infants have gained roughly 50% more than their birth weight, which is one of the fastest growth spurts they’ll ever experience.
Average Weight and Length at 3 Months
During the first few months of life, babies gain about 1 ounce per day, or roughly 2 pounds per month. That means a baby born at an average 7.5 pounds will typically weigh somewhere between 12 and 14 pounds by the 3-month mark. Length increases at a rate of about three-quarters of a centimeter per week in this window, adding roughly 3 to 4 inches to their birth length by month three.
These are averages pulled from the middle of the growth curve. A baby at the 25th percentile and a baby at the 75th percentile can differ by several pounds and still be perfectly healthy. What pediatricians care about most isn’t a single measurement but whether your baby is following a consistent curve over time. A smaller baby who stays on the 15th percentile is growing normally. A baby who drops from the 50th to the 10th percentile over a couple of visits might need a closer look.
Breastfed vs. Formula-Fed Differences
Breastfed and formula-fed babies tend to be similar in size at 3 months, but their growth patterns start to diverge right around this point. Breastfed infants typically put on weight more slowly than formula-fed infants after the 3-month mark, according to CDC growth data. Formula-fed babies gain weight more quickly from here on out. Their length, however, tracks similarly regardless of feeding method.
This matters because your pediatrician should be using the right growth chart. The WHO charts, which are based primarily on breastfed infants, are recommended for all babies under 2. If your breastfed baby looks like they’re “falling behind” on an older CDC chart designed around formula-fed norms, that difference in chart design could be the explanation rather than an actual growth problem.
What Clothing and Diaper Size to Expect
Most 3-month-olds are transitioning out of 0-3 month clothes and into the 3-6 month range. Standard 0-3 month sizing fits babies 9 to 12.5 pounds and 21.5 to 24 inches. The 3-6 month size covers 12.5 to 17 pounds and 24 to 26.5 inches. If your baby is right at 3 months, there’s a good chance they’re straddling the line between these two sizes, so buying 3-6 month clothes is the safer bet.
For diapers, most 3-month-olds fit a size 1 (8 to 14 pounds) or size 2 (12 to 18 pounds). Weight matters far more than age when choosing diaper size. If your baby is between sizes, go up rather than down. The larger size absorbs more and covers better, which helps with leaks, especially at night.
Physical Changes Beyond the Numbers
Size at 3 months isn’t just about weight and length. Your baby’s body proportions and muscle tone are visibly different from the newborn stage. By the end of month three, most babies can lift their head and chest off the floor during tummy time, propping themselves up on their elbows. They can support their own head when held upright, which typically starts around 2 months and feels much steadier by 3 months.
Their hands are opening and closing more deliberately now, and many 3-month-olds can grab toys and bring them to their mouth. The baby you’re holding at 3 months feels sturdier and more solid than a newborn, partly because of the added weight but also because of real gains in muscle control. Head circumference is also growing at about half a centimeter per week during this period, reflecting rapid brain development underneath.
Growth Expectations for Premature Babies
If your baby was born early, size expectations at 3 months look different. Pediatricians use “corrected age” rather than chronological age when plotting a preemie’s growth. Corrected age subtracts the weeks of prematurity from the baby’s actual age. So a baby born 6 weeks early who is now 3 months old would be evaluated as a 6-week-old on the growth chart. This adjustment is used until age 2.
Premature babies typically follow the same weekly growth rates as full-term babies once they reach their due date: about three-quarters of a centimeter per week in length and half a centimeter per week in head circumference during the first 3 months of corrected age. They’re on the same trajectory, just shifted by the number of weeks they arrived early. A 3-month-old preemie who seems small for their chronological age may be right on track for their corrected age.