The average 4-month-old is about 24 to 25 inches long (61 to 64 cm), with boys typically measuring slightly longer than girls. On the WHO growth charts used by pediatricians, the 50th percentile for a 4-month-old boy is roughly 25.2 inches (63.9 cm), and for a girl it’s about 24.4 inches (62.1 cm). But “average” is just the midpoint of a wide healthy range, and where your baby falls on the growth chart matters less than whether they’re growing consistently over time.
Average Length by Sex
Boys and girls follow slightly different growth curves from birth. At 4 months, the typical ranges on the WHO growth charts look like this:
- Boys: Most fall between 23.2 and 27 inches (59 to 68.6 cm), covering the 3rd to 97th percentile. The 50th percentile is about 25.2 inches (63.9 cm).
- Girls: Most fall between 22.4 and 26.2 inches (57 to 66.6 cm). The 50th percentile is about 24.4 inches (62.1 cm).
A baby at the 15th percentile is just as healthy as one at the 85th. These numbers simply describe how your baby’s length compares to other babies the same age. The AAP and CDC both recommend using the WHO growth charts for children under 2 years old, since those charts are based on data from breastfed infants across multiple countries and represent how babies grow under optimal conditions.
Growth Rate Matters More Than a Single Number
Babies grow roughly 1 inch (2.5 cm) per month during the first six months of life. That pace is remarkably fast compared to later infancy, when growth slows to about half an inch per month between 6 and 12 months. A baby born at 20 inches who’s now 24 inches at 4 months is right on track with that expected pattern.
Pediatricians pay more attention to a baby’s growth curve, the trajectory of measurements plotted visit after visit, than to any single number. A baby who’s been tracking along the 20th percentile since birth is growing normally. What raises concern is when a baby’s growth rate drops across two major percentile lines (for example, sliding from the 75th down to the 25th). That kind of shift can signal a feeding issue, an underlying health condition, or simply a temporary slowdown that resolves on its own. Your pediatrician will look at the pattern across several visits before drawing conclusions.
What Affects Your Baby’s Length
Genetics plays a significant role, but it doesn’t fully explain infant length the way it does adult height. In the first two years of life, environmental factors like nutrition, feeding practices, and overall health can overshadow genetic potential. That’s why two parents of very different heights might have a baby who lands in the middle of the chart, or why a baby can temporarily grow faster or slower than their “genetic target” based on how well they’re feeding.
Maternal height has a particularly strong influence on early growth. Shorter mothers may produce less breast milk volume or have had nutritional constraints during pregnancy that affect birth size, giving their babies a different starting point on the growth curve. Paternal height tends to become more influential later in childhood. By 2 to 3 years old, most children start settling into a growth pattern that better reflects their parents’ combined height.
Why Measuring at Home Is Tricky
If you’ve tried measuring your baby at home and gotten a number that seems off, the measurement itself may be the problem. Babies are measured lying flat on their backs using a rigid measuring board with a fixed headpiece and a movable footboard. The baby’s legs need to be fully extended with toes pointing upward, and someone needs to hold the baby’s head gently against the headboard while the measurer slides the footboard snug against both heels. Getting a wiggly 4-month-old to cooperate with all of that is hard enough with two trained people in a clinic.
Measuring on a soft surface, using a flexible tape, or not fully straightening the legs can easily produce readings that are off by an inch or more. If you’re curious about your baby’s length between checkups, it’s fine to try at home, but treat the result as a rough estimate rather than a precise data point.
Adjusted Age for Premature Babies
If your baby was born before 37 weeks, the standard 4-month benchmarks don’t apply directly. Premature babies should be measured against their corrected age, which is calculated by subtracting the weeks of prematurity from their actual age. A baby born at 34 weeks (6 weeks early) who is now 4 months old has a corrected age of about 2.5 months, and their length should be compared to the growth chart values for that corrected age.
Skipping this adjustment leads to dramatically misleading results. At 4 months, about 40% of very premature infants who are growing appropriately would be wrongly flagged as growth-faltering if measured against their calendar age. One study found that more than twice as many preterm children were classified as stunted at 4 months when corrected age wasn’t used (66% vs. 28%). Most pediatricians continue using corrected age until at least 24 months for preterm babies, and sometimes longer for those born very early.
What the 4-Month Checkup Covers
The 4-month well visit is one of the routine appointments where your pediatrician will measure length, weight, and head circumference, then plot all three on the growth chart. They’re looking at how these three measurements relate to each other and whether your baby’s curve is following a consistent trajectory. A baby who is long but light might be assessed differently than one who is short and heavy, because the proportional relationship between length and weight tells its own story.
If your baby’s length is below the 3rd percentile, or if they’ve crossed two major percentile lines downward since their last visit, your pediatrician will likely want to investigate. That could mean reviewing feeding patterns, checking for signs of illness or absorption problems, or simply scheduling a follow-up visit in a few weeks to see if the trend continues. Most of the time, babies who dip temporarily bounce back without intervention.