The average full-term newborn weighs about 7 pounds (3.2 kg) and measures roughly 19.75 inches (50 cm) long. Head circumference, the other standard measurement taken at birth, averages 13.75 inches (35 cm). These numbers represent the midpoint, though healthy babies come in a wide range of sizes depending on genetics, gestational age, and several maternal factors.
Weight, Length, and Head Circumference
Hospitals measure three things within minutes of birth: weight, length, and head circumference. Weight gets the most attention because it’s the easiest number to track over the coming days and weeks, but all three measurements help determine whether a baby falls within the expected range for their gestational age.
A baby born at 40 weeks who weighs over about 8 pounds 13 ounces (4,000 g) falls at or above the 90th percentile, a classification called “large for gestational age.” On the other end, babies born under 5 pounds 8 ounces (2,500 g) are classified as low birth weight. Both extremes can signal the need for closer monitoring, but they don’t automatically mean something is wrong.
How Size Changes Week by Week
“Full term” covers a five-week window, from 37 to 42 weeks of gestation, and babies gain meaningful weight during each of those weeks. Data from nearly 3.75 million singleton births in Poland illustrates the pattern clearly:
- 37 weeks: Boys average about 6 lbs 14 oz (3,124 g); girls about 6 lbs 9 oz (2,987 g)
- 38 weeks: Boys about 7 lbs 6 oz (3,348 g); girls about 7 lbs 1 oz (3,201 g)
- 39 weeks: Boys about 7 lbs 12 oz (3,501 g); girls about 7 lbs 6 oz (3,348 g)
- 40 weeks: Boys about 8 lbs (3,622 g); girls about 7 lbs 10 oz (3,465 g)
- 41 weeks: Boys about 8 lbs 3 oz (3,714 g); girls about 7 lbs 13 oz (3,552 g)
- 42 weeks: Boys about 8 lbs 4 oz (3,751 g); girls about 7 lbs 15 oz (3,589 g)
That’s roughly a pound and a half of growth between 37 and 42 weeks. A baby born at 37 weeks who seems small may simply need the extra growing time that a 40-week baby already had.
Boys vs. Girls
Male newborns are consistently heavier and slightly longer than female newborns at every gestational age. The difference is modest, typically around 150 to 160 grams (about 5 ounces) in weight and roughly half a centimeter in length. This gap is large enough to show up in population data but small enough that it rarely matters in practice. Pediatricians use sex-specific growth charts for exactly this reason.
What Influences Birth Size
Gestational age and biological sex are the two biggest predictors, but several other factors play a role. Taller and heavier mothers tend to have larger babies, which makes intuitive sense since the baby’s growth environment is physically larger. Maternal nutrition during pregnancy matters too: women with higher body mass and adequate caloric intake generally deliver heavier babies, while nutritional deficiencies can reduce birth weight.
Gestational diabetes is one of the most well-known drivers of above-average birth size. When a mother’s blood sugar runs high, the baby receives more glucose than it needs and stores the excess as fat, sometimes pushing birth weight above the 90th percentile. Babies over 4,000 g (about 8 lbs 13 oz) or, by some definitions, over 4,500 g (9 lbs 15 oz) are considered macrosomic.
Other factors that can shift birth weight include maternal age (each additional year of maternal age is associated with a slight decrease in birth weight), whether the mother has had previous pregnancies (later babies tend to be somewhat larger than first babies), and the spacing between pregnancies. Even family income plays a measurable role, likely because it correlates with nutrition, stress levels, and access to prenatal care.
Normal Weight Loss After Birth
New parents are often caught off guard when their baby weighs less at discharge than at birth. This is completely normal. Breastfed newborns lose an average of 6 to 7 percent of their birth weight before they start gaining, with the lowest point typically hitting around 38 to 39 hours after delivery. For a 7-pound baby, that’s a loss of about 6 to 7 ounces.
Weight gain usually begins by around day 3, and most babies return to their birth weight within 10 to 14 days. The hospital will weigh your baby before discharge and your pediatrician will check again within the first week to make sure the pattern is on track. A loss greater than 10 percent of birth weight warrants a closer look at feeding.
What This Means for Clothing Sizes
If you’re shopping for a newborn, the numbers above explain why “newborn” sized clothing is surprisingly small. Most brands cut newborn sizes for babies up to about 7 to 8 pounds and 20 inches long. Since the average baby is right at 7 pounds and just under 20 inches, many newborns fit into that size for only a week or two before moving into 0 to 3 month clothing, which generally covers babies from about 8 to 12 pounds.
Babies born at 40 or 41 weeks sometimes skip newborn sizes entirely. If you know your due date and your late ultrasound estimates suggest a larger baby, stocking up on 0 to 3 month clothes is a safer bet. Most clothing will list a weight and height range on the tag, which is more reliable than the age label since babies vary so much in size.