The average full-term newborn weighs about 7 pounds (3.2 kg) and measures roughly 19.75 inches (50 cm) long. Most healthy babies fall within a range around these numbers, but the exact size your baby lands at depends on several factors, from genetics and sex to how many weeks of pregnancy passed before delivery.
Average Weight and Length
A full-term baby, born between 37 and 41 weeks of gestation, typically weighs between 5.5 and 10 pounds, with 7 pounds being the midpoint. For length, 50 cm (just under 20 inches) is the standard average. Head circumference at birth usually falls around 13.5 inches (34 cm).
These numbers describe the middle of a bell curve. Babies who are somewhat lighter or heavier are usually perfectly healthy. The World Health Organization growth charts place a newborn girl at roughly 2.5 kg (5.5 lbs) at the 5th percentile and 4.0 kg (8.8 lbs) at the 95th percentile, so there’s a wide window of normal.
Boys vs. Girls
Male newborns are consistently a bit bigger than females. Research from a cohort of over 1,000 births found that boys have both a higher birth weight and a greater birth length on average. The difference isn’t dramatic, but it’s consistent enough that growth charts are separated by sex. At 40 weeks, for example, the average male singleton weighs about 3,638 grams (8.0 lbs), while the average female singleton weighs about 3,486 grams (7.7 lbs), a gap of roughly 150 grams or a third of a pound.
How Gestational Age Changes the Numbers
Every additional week in the womb adds meaningful weight. A baby born at 37 weeks is technically full-term, but will be noticeably smaller than one born at 40 weeks. Population data from Canada illustrates this clearly:
- 37 weeks: Boys average 3,091 g (6.8 lbs); girls average 2,981 g (6.6 lbs)
- 38 weeks: Boys average 3,306 g (7.3 lbs); girls average 3,181 g (7.0 lbs)
- 39 weeks: Boys average 3,489 g (7.7 lbs); girls average 3,350 g (7.4 lbs)
- 40 weeks: Boys average 3,638 g (8.0 lbs); girls average 3,486 g (7.7 lbs)
That’s roughly 200 grams (about half a pound) gained per week in the final stretch. This is why doctors pay close attention to gestational age when evaluating whether a baby’s size is on track.
What Counts as Too Small or Too Large
Doctors classify babies as “small for gestational age” when their weight falls below the 10th percentile for their specific week of birth. On the other end, “large for gestational age” generally means above the 90th percentile. Neither label automatically means something is wrong, but both prompt extra monitoring.
A small-for-gestational-age baby born at 39 weeks, for instance, isn’t just being compared to the general 7-pound average. The comparison is against other 39-week babies of the same sex. This is a more precise way to spot potential growth issues than using a single cutoff weight.
Factors That Influence Birth Size
Genetics plays the biggest background role. Taller, larger parents tend to have bigger babies, and this pattern holds across populations. But several modifiable factors also shift the numbers.
Smoking during pregnancy has one of the most well-documented effects on birth weight. Active smoking reduces birth weight in a dose-dependent way, meaning more cigarettes lead to greater reductions. Research measuring carbon monoxide levels in pregnant women found that moderate smokers had babies averaging 350 grams (about 12 ounces) lighter than nonsmokers. Heavy smokers saw reductions as large as 755 grams (1.7 lbs). Each cigarette smoked during the third trimester is associated with roughly a 27-gram decrease in birth weight. Passive exposure matters too: having a smoking partner at home doubles the risk of a low-birth-weight baby. The encouraging finding is that quitting before the end of the first trimester brings birth weight back to the same level as babies of nonsmoking mothers.
Gestational diabetes pushes birth weight in the opposite direction. When a mother’s blood sugar runs high, the baby receives extra glucose and stores it as fat, often resulting in a larger-than-average infant. Nutrition, maternal blood pressure, and whether the pregnancy involves multiples (twins gain less weight individually) also play significant roles.
Why Birth Size Matters
A baby’s size at birth gives doctors an early snapshot of how well growth went in the womb. Babies on the very small end face higher risks of blood sugar drops and temperature regulation problems in the first hours of life. Very large babies can complicate delivery and are more likely to experience temporary blood sugar issues of their own.
That said, birth weight is just a starting point. Pediatricians track growth over weeks and months using percentile curves, and what matters most is that a baby follows a consistent trajectory. A baby born at the 25th percentile who stays near the 25th percentile is growing normally. Sudden jumps or drops across percentile lines get more attention than the initial number on the scale.