The average newborn weighs about 7.5 pounds (3.5 kg) and measures 20 inches (50 cm) long. Most full-term babies fall within a wide healthy range, though, so there’s no single “right” size. Anything from 5.5 to 10 pounds and 18 to 22 inches is considered normal.
Average Weight and Length at Birth
A full-term baby, born between 39 and 40 weeks, typically weighs between 5.5 and 10 pounds (2.5 to 4.5 kg), with the average sitting right around 7.5 pounds. For length, the normal range runs from about 18 to 22 inches (45.7 to 56 cm), centered on an average of 20 inches.
These ranges are broad for a reason. A healthy 6-pound baby and a healthy 9-pound baby can both be perfectly normal. Doctors pay more attention to whether a baby’s weight is proportional to their length and gestational age than to any single number.
How Size Changes by Week of Pregnancy
Babies gain weight rapidly in the final weeks of pregnancy. A baby born even a few weeks early will typically be noticeably smaller than one born at full term. Here’s how average size tracks through the third trimester:
- 32 weeks: About 3.75 pounds (1,700 g) and 11 inches from crown to rump
- 34 weeks: About 4.5 pounds (2,100 g) and nearly 12 inches from crown to rump
- 38 weeks: About 6.5 pounds (2,900 g), though some babies already weigh close to 9 pounds at this point
- 40 weeks (full term): About 7.5 pounds (3,500 g) on average
Those crown-to-rump measurements only capture the baby curled up, not at full stretch. Total body length at birth, measured head to heel, is the 18-to-22-inch range mentioned above. The key takeaway is that babies roughly double their weight between 32 weeks and full term, which is why every extra week of pregnancy matters so much for growth.
When Babies Are Considered Too Small or Too Large
Babies born weighing less than 5.5 pounds (2,500 g) are classified as low birth weight. This is most common in premature babies, but it can also happen in full-term infants who didn’t grow as expected in the womb. Low birth weight babies often need extra monitoring and sometimes time in a neonatal unit to help with feeding and temperature regulation.
On the other end, a baby weighing more than 8 pounds 13 ounces (4,000 g) is often described as larger than average, and the medical term “macrosomia” kicks in at either that threshold or at 9 pounds 15 ounces (4,500 g), depending on the guidelines being used. Larger babies can make delivery more complicated, sometimes leading to a longer labor or a higher likelihood of cesarean birth. Above 11 pounds (5,000 g) is rare and carries more significant delivery risks.
What Determines a Baby’s Size
Birth size isn’t random. It’s shaped by a combination of factors, some within a parent’s control and some not.
Genetics plays an obvious role. Taller, larger parents tend to have bigger babies. The mother’s own birth weight is one of the stronger predictors. Sex matters too: boys are, on average, slightly heavier than girls at birth.
Maternal health during pregnancy has a major influence. A mother’s weight, height, and nutrition all correlate with birth weight. Anemia during pregnancy (low iron levels in the blood) is linked to smaller babies, as is smoking or inadequate weight gain. Gestational diabetes, on the other hand, often produces larger-than-average babies because the baby is exposed to higher blood sugar levels in the womb.
Other factors include the spacing between pregnancies (shorter intervals are associated with lower birth weight), the mother’s age, and family income, which often serves as a proxy for access to nutrition and prenatal care. First babies also tend to be slightly smaller than subsequent siblings.
What Babies Weigh in the First Days
Nearly all newborns lose weight in the first few days after birth. This is normal and expected. Most babies drop 5% to 10% of their birth weight within the first week, primarily from losing excess fluid. A 7.5-pound baby might dip to around 7 pounds before turning the corner.
Most babies regain their birth weight by 10 to 14 days of age, then continue gaining roughly 5 to 7 ounces per week for the first few months. So the number on the scale at birth is really just a starting point. Pediatricians track growth over weeks and months using percentile charts that compare your baby’s trajectory to thousands of other infants of the same age and sex. Consistent growth along a curve matters more than where on that curve your baby falls.