The average newborn measures about 19 to 20 inches (48 to 51 cm) long at birth. Boys tend to be slightly longer than girls. According to WHO growth standards used by the CDC, the 50th percentile birth length is 50.5 cm (19.9 inches) for boys and 49.9 cm (19.6 inches) for girls. Most full-term babies fall within an inch or so of those numbers.
Average Length for Boys vs. Girls
The difference between newborn boys and girls is small but consistent. Boys average about half a centimeter longer at birth. In practical terms, most full-term newborns of either sex measure between 18 and 22 inches. Babies at the 5th percentile (shorter than 95% of newborns) are around 18 inches, while those at the 95th percentile reach roughly 21.5 inches.
It’s worth noting that doctors measure newborns lying down, not standing up. This “recumbent length” uses a flat board with a fixed headpiece and a moveable footpiece pressed gently against the baby’s feet. Because newborns naturally curl their legs, getting an accurate measurement takes a bit of effort, and numbers can vary slightly between attempts. That’s why your baby’s birth length might differ a little from what you see at a follow-up visit.
What Determines a Newborn’s Length
Genetics is the single biggest factor. A large study using parent-offspring data found that the baby’s own genes explain about 31% of the normal variation in birth length, while the mother’s genes independently account for another 19%. That means roughly half of the variation in how long a baby is at birth traces back to genetic factors. Tall parents are more likely to have longer babies, though it’s far from a perfect predictor.
Maternal health during pregnancy matters too. Mothers who ate extra meals during pregnancy, had healthy iron levels (hemoglobin above 11 g/dL), and got adequate rest were significantly more likely to deliver babies measuring above 48 cm. Conditions like high blood pressure, gestational diabetes, and chronic stress can all reduce a baby’s length at birth. Nutrient intake is especially influential: the building blocks for bone and tissue growth come directly from what the mother consumes over nine months.
How Premature Babies Compare
Gestational age has a dramatic effect on length. A baby born at 28 weeks typically measures around 15 inches. At 32 weeks, that increases to about 17 inches. By 36 weeks (considered “late preterm”), babies average around 19 inches, approaching full-term size. Each additional week in the womb adds measurable length, which is one reason doctors try to delay preterm delivery when it’s safe to do so.
These shorter measurements aren’t a sign of a problem on their own. Premature babies are expected to be smaller, and pediatricians track their growth using “corrected age,” adjusting for how early they arrived, until they catch up.
When Birth Length Is a Concern
Doctors flag a newborn as “small for gestational age” (SGA) when birth length or weight falls below a specific threshold: more than 2 standard deviations below the average for their gestational age. For a full-term baby, that roughly translates to a length under about 18 inches (45.5 cm). A baby can be SGA for weight, length, or both.
Being born small doesn’t automatically signal a lasting issue. Most SGA babies experience catch-up growth within the first two years. Pediatricians monitor these children more closely during infancy to make sure growth is progressing on track, and the vast majority reach a normal range without any intervention.
How Fast Newborns Grow
Newborns grow remarkably fast once they’re out of the womb. From birth through about six months, babies gain roughly 1 inch (2.5 cm) in length per month. That pace means a 20-inch newborn could be around 26 inches by their half-birthday. Growth slows after six months but continues at a steady clip through the first year.
Birth length is a less reliable predictor of adult height than you might expect. The correlation strengthens over time, but a baby’s length at two years old is a much better indicator of where they’ll end up. Many babies who are shorter or longer than average at birth shift toward the middle of the growth curve within their first year as their own genetic programming takes over from the uterine environment.