A full-term newborn weighs about 7.5 pounds (3.5 kg) on average and measures around 20 inches (50 cm) long. Most healthy babies fall within a range, though, and several factors determine where your baby lands on that spectrum.
Average Weight and Length at Birth
The average birth weight of 7.5 pounds is just the midpoint. Most full-term babies weigh between 5.5 and 10 pounds, and anything in that range is considered normal. Length at birth averages 20 inches but commonly falls between 18 and 22 inches. Head circumference, which your baby’s doctor will also measure, typically runs about 13.5 inches (34 cm).
Boys tend to be slightly larger than girls at birth. At 40 weeks of gestation, the median weight for a boy is about 7 pounds 15 ounces (3,613 g), while the median for a girl is roughly 7 pounds 14 ounces (3,576 g). The difference is small but consistent across populations.
How Gestational Age Changes Size
Babies gain weight rapidly in the final weeks of pregnancy, so even a week or two of difference in timing can shift their size noticeably. A boy born at 37 weeks (the earliest point still considered “early term”) has a median weight of about 6 pounds 12 ounces (3,080 g). By 40 weeks, that jumps to nearly 8 pounds. Girls follow a similar pattern, going from about 7 pounds at 37 weeks to 7 pounds 14 ounces at 40 weeks.
Here’s roughly how median birth weight increases week by week for boys:
- 37 weeks: 6 lb 12 oz (3,080 g)
- 38 weeks: 7 lb 4 oz (3,290 g)
- 39 weeks: 7 lb 10 oz (3,465 g)
- 40 weeks: 7 lb 15 oz (3,613 g)
- 41 weeks: 8 lb 3 oz (3,733 g)
Babies born before 37 weeks are classified as preterm and are often significantly smaller. A baby born at 34 weeks, for example, may weigh only 4 to 5 pounds. Premature babies have their growth tracked on adjusted charts that account for how early they arrived.
When a Baby Is Considered Too Large or Too Small
Doctors use specific thresholds to flag babies whose size may pose health risks. A baby weighing more than 8 pounds 13 ounces (4,000 g) is considered large for gestational age, a condition called macrosomia. Health risks increase further when weight exceeds 9 pounds 15 ounces (4,500 g), including a higher chance of complications during delivery for both the baby and the mother.
On the other end, babies born weighing less than 5 pounds 8 ounces (2,500 g) are classified as low birth weight. This is most commonly linked to preterm delivery but can also happen when a full-term baby hasn’t grown as expected in the womb, a situation sometimes called intrauterine growth restriction.
What Determines Your Baby’s Size
Several factors work together to influence how big or small a baby is at birth. Genetics plays a large role: taller, larger parents tend to have bigger babies, and a mother’s own birth weight is a surprisingly strong predictor of her baby’s size.
Beyond genetics, some of the most significant factors include:
- Gestational age: This is the single biggest determinant. Babies born even a few weeks early are substantially lighter, while post-due-date babies tend to be heavier.
- Maternal nutrition and body weight: Mothers with a very low BMI (under 18.5) have a significantly higher chance of delivering a smaller baby. Conversely, gestational diabetes or higher pre-pregnancy weight is associated with larger babies.
- Blood pressure during pregnancy: Pregnancy-related high blood pressure roughly doubles the odds of a baby being born at a lower weight, because it can restrict blood flow through the placenta.
- Number of previous pregnancies: First babies tend to be slightly smaller than later siblings. However, mothers who have had many pregnancies (five or more) may also see smaller birth weights.
- Multiples: Twins, triplets, and other multiples almost always weigh less individually than singletons, partly because they share space and nutrients and partly because they tend to arrive earlier.
Smoking during pregnancy, alcohol use, and certain chronic health conditions like kidney disease can also reduce birth weight. On the flip side, well-managed prenatal care and adequate nutrition are linked to babies landing closer to the healthy average.
Weight Loss in the First Few Days
Almost all newborns lose weight in the first few days after birth, which can surprise new parents. This is normal. Babies are born with extra fluid that they shed quickly, and they’re taking in only small amounts of colostrum or formula while feeding gets established.
Most babies start regaining weight between days 3 and 5, and about 80 percent are back to their birth weight by 2 weeks of age. A loss of up to 7 percent of birth weight is typical. If a baby loses more than 10 percent, or is slow to regain, that signals a need for closer attention to feeding. For a 7.5-pound baby, 10 percent works out to about 12 ounces, so even modest-sounding numbers matter at this scale.
How Newborn Size Compares to Everyday Objects
If you’re trying to picture what 7.5 pounds and 20 inches actually looks like, it helps to think in familiar terms. A newborn is roughly the length of a standard laptop screen, or about the distance from your elbow to your fingertips. In weight, a newborn is comparable to a gallon of milk or a small bowling ball. Their head makes up about a quarter of their total body length, which is a much bigger proportion than in adults and part of why newborns look so distinctive.
By the end of the first month, most babies have gained about 1.5 to 2 pounds and grown roughly an inch. By 5 months, the average baby has doubled their birth weight. By their first birthday, most have tripled it, weighing around 22 pounds and measuring about 30 inches long.