A full-term newborn weighs about 3.5 kg (7.5 lb) on average and measures roughly 50 cm (20 in.) long. Most healthy babies fall within a range of 2.5 to 4.5 kg (5.5 to 10 lb) and 45.7 to 56 cm (18 to 22 in.) at birth. That’s a wide spread, and where your baby lands within it depends on a mix of genetics, sex, and pregnancy-related factors.
Average Weight and Length at Birth
The 7.5 lb average is a useful benchmark, but the normal range spans nearly five pounds from smallest to largest. A 5.5 lb baby and a 10 lb baby can both be perfectly healthy full-term newborns. Length follows a similar pattern: 20 inches is typical, but anything from 18 to 22 inches falls within the expected range.
Babies born significantly smaller or larger than average get classified differently by healthcare providers. Infants whose weight falls below the 10th percentile for their gestational age are considered “small for gestational age,” while those above the 90th percentile are “large for gestational age.” These labels don’t automatically signal a problem, but they do prompt closer monitoring to make sure the baby is feeding well and growing on track.
Boys vs. Girls
Male newborns tend to be slightly heavier than females. In a large statistical analysis, boys averaged 3.34 kg compared to 3.26 kg for girls, a difference of about 85 grams (roughly 3 ounces). That gap is small but statistically consistent across normal-weight births. Among babies born at higher weights (above 4 kg), the difference between boys and girls largely disappears.
What Determines a Baby’s Size
Genetics is the single biggest factor. Research estimates that genetic influences account for 38 to 80 percent of the variation in birth weight, while environmental factors (nutrition, altitude, maternal health) explain about 25 percent. The rest comes down to complex interactions between these forces during pregnancy.
Both parents contribute genetically. A mother’s genes affect birth size partly through how her body manages blood sugar and delivers nutrients through the placenta. A father’s genes influence fetal growth more directly, through the baby’s own genetic blueprint. One well-studied example involves a gene related to blood sugar regulation: when the mother carries a specific variant, her higher blood sugar during pregnancy pushes the baby’s weight up by about 600 grams. When the baby inherits that same variant, it actually reduces birth weight by about 530 grams, enough to shift from the 50th percentile down to the 25th. Parent and baby genetics can push in opposite directions.
The placenta plays its own role. It controls how much nutrition reaches the baby and acts as a barrier against infection and stress hormones. In about 1 to 2 percent of pregnancies, the placenta develops genetic differences from the fetus itself. This mismatch accounts for up to 20 percent of cases where full-term babies are unexpectedly small.
Several growth-related hormones are critical during fetal development. Insulin and insulin-like growth factors are among the most important signals that drive a baby’s size in the womb. Rare disruptions to these pathways can lead to significant growth restriction, but for most pregnancies, it’s the collective effect of many small genetic contributions, combined with maternal nutrition and health, that lands a baby at a particular weight.
Twins and Multiples Are Smaller
Twins are almost always lighter than singletons at birth. This is largely because they arrive earlier on average and because they share the uterine space and placental resources. A large study comparing over 700 twins to more than 40,000 singletons confirmed that both male and female twins had lower birth weights. Female twins caught up to singletons of similar gestational age by about 15 months, but still hadn’t fully closed the gap with the broader singleton population by age two. Twins tend to gain weight rapidly in infancy, gradually making up the difference.
Weight Loss in the First Week
Almost all newborns lose weight in the first few days of life. This is normal. Babies are born with extra fluid, and they’re still learning to feed efficiently. Most begin regaining weight between days three and five, and 80 percent are back to their birth weight by two weeks of age.
A loss of up to about 7 percent of birth weight is typical and not a concern. For a 7.5 lb baby, that’s a little over half a pound. If weight loss reaches 10 percent or more, or if the baby is slow to start gaining, that signals a need for closer evaluation, usually focused on whether feeding is going well. Breastfed babies sometimes take a day or two longer to start gaining compared to formula-fed babies, because mature breast milk takes a few days to come in fully.
What “Normal” Really Means
Birth size is one data point, not a verdict. A baby born at 6 pounds can be just as healthy as one born at 9 pounds. What matters more than the number on the scale at birth is the growth trajectory in the weeks and months that follow. Pediatric growth charts track weight, length, and head circumference over time, and the trend line is far more informative than any single measurement. A baby who is consistently growing along their own curve, whether that’s the 15th percentile or the 85th, is doing exactly what they should be.