At 4 months old, most babies weigh between 12 and 17 pounds and measure around 24 to 26 inches long. By this age, many infants have doubled their birth weight, which is one of the first major growth milestones pediatricians track. Of course, healthy babies come in a wide range of sizes, so these numbers represent averages rather than targets every baby needs to hit.
Average Weight and Length at 4 Months
A typical 4-month-old gains about 1 to 1.25 pounds per month and grows roughly half an inch to one inch in length each month. Head circumference also increases by about half an inch per month during this period. These gains add up quickly. A baby born at 7 pounds, for instance, will often weigh 14 pounds or more by the four-month mark.
Boys tend to be slightly heavier and longer than girls at this age, but the overlap is significant. What matters more than hitting an exact number is that your baby is following a consistent curve on their growth chart. A baby in the 25th percentile who stays near the 25th percentile is growing perfectly well.
Breastfed vs. Formula-Fed Growth
Growth patterns start to diverge around 3 months depending on how a baby is fed. Formula-fed infants typically gain weight more quickly after 3 months of age, while breastfed babies tend to put on weight more slowly through the first year. This is normal and expected. The CDC notes that length growth is similar regardless of feeding method, so the difference is primarily in weight gain pace, not overall development.
If your baby is breastfed and seems lighter than formula-fed peers, that alone isn’t a concern. Pediatricians use growth charts specifically designed for breastfed infants to account for these differences.
What Size Diapers and Clothes Fit
Most 4-month-olds are transitioning from size 1 diapers (designed for 8 to 14 pounds) into size 2 diapers (designed for 12 to 18 pounds). Weight is the real guide here, not age. If diapers are leaking at the waistband or leaving red marks on your baby’s thighs, it’s time to size up regardless of what the package says about age range.
For clothing, most 4-month-olds fit into the “3 to 6 months” or “6M” size, which typically corresponds to 12.5 to 17 pounds and 24 to 26.5 inches in length. Sizing varies between brands, so checking the weight and height ranges on the label is more reliable than going by the month label alone. Many parents find their babies outgrow the month label before they actually reach that age, especially for longer or heavier babies.
Physical Development at This Size
At 4 months, a baby’s size supports some exciting physical changes. Their neck muscles are strong enough to balance their head well during tummy time and while being held upright. Many babies can push up with straight arms when placed on their stomachs, a precursor to rolling over (which most babies accomplish by 6 months).
These milestones are connected to overall growth. Bigger muscles and a more proportional head-to-body ratio give babies the strength and balance they need to start moving more deliberately. Plenty of tummy time helps them build on the physical foundation their growth provides.
When Growth Seems Off Track
Pediatricians watch for two main patterns that signal a potential issue: a baby who crosses two or more percentile lines on the growth chart (either up or down), or a baby whose weight and length are dramatically mismatched, such as very high weight for a much lower length percentile. A single measurement that looks high or low is rarely meaningful on its own. The trend over multiple visits tells the real story.
Premature babies often track smaller than the standard charts suggest at 4 months. Their growth is typically measured using an adjusted age, which accounts for the weeks they missed in the womb. A baby born 6 weeks early, for example, would be compared against the size expectations for a 2.5-month-old rather than a 4-month-old.