A full-term newborn typically weighs between 5 pounds 8 ounces and 8 pounds 13 ounces (2,500 to 4,000 grams), with an average right around 7 pounds 5 ounces (3,300 grams). Most babies measure between 19 and 21 inches long (48 to 53 centimeters). Those numbers shift depending on the baby’s sex, how many weeks the pregnancy lasted, and several maternal health factors.
Average Weight and Length
Babies below 5 pounds 8 ounces are classified as low birth weight, while those above 8 pounds 13 ounces are considered high birth weight. The vast majority of full-term babies land somewhere in between. Head circumference at birth typically falls between 13 and 14.5 inches (33 to 37 centimeters), and that measurement is one of the first things checked in the delivery room because it reflects early brain growth.
These ranges apply to babies born between 37 and 42 weeks of pregnancy. A baby born earlier will generally be smaller, and a baby born later in that window will be larger. Twins and other multiples tend to weigh less than singletons, even when born at the same gestational age.
How Size Changes Week by Week
Babies gain a surprising amount of weight in the final weeks of pregnancy. Canadian population data on singleton births shows how much size shifts between 37 and 40 weeks:
- 37 weeks: Average weight around 6 pounds 12 ounces (3,080 g for boys, 2,968 g for girls)
- 38 weeks: Average weight around 7 pounds 2 ounces (3,290 g for boys, 3,169 g for girls)
- 39 weeks: Average weight around 7 pounds 8 ounces (3,465 g for boys, 3,334 g for girls)
- 40 weeks: Average weight around 7 pounds 14 ounces (3,613 g for boys, 3,470 g for girls)
That means a baby gains roughly a pound between 37 and 40 weeks. This is one reason doctors prefer pregnancies to reach at least 39 weeks when possible: those last two weeks add meaningful body weight, which helps with temperature regulation and feeding after birth.
Boys vs. Girls
Male newborns are consistently a bit heavier than females. On average, boys weigh about 3,343 grams (7 pounds 6 ounces) compared to 3,258 grams (7 pounds 3 ounces) for girls. That gap of roughly 3 ounces holds across the normal weight range and gets slightly wider among high birth weight babies, where boys average about 4,462 grams and girls about 4,342 grams.
The difference is modest but reliable enough that growth charts are split by sex. If your pediatrician mentions percentiles at a checkup, they’re comparing your baby’s size against other babies of the same sex and age.
What Affects Birth Size
A number of factors influence how big a baby is at birth, some modifiable and some not:
- Genetics: Taller, larger parents tend to have bigger babies. This is the single strongest predictor of birth size.
- Gestational diabetes: High blood sugar during pregnancy sends extra glucose to the baby, which can cause them to grow larger than average (a condition sometimes called macrosomia, defined as birth weight above 4,000 grams or about 8 pounds 13 ounces).
- Maternal nutrition: Undernutrition or poor weight gain during pregnancy is one of the leading contributors to low birth weight worldwide.
- Smoking and substance use: Smoking during pregnancy restricts blood flow to the placenta and consistently reduces birth weight.
- Birth order: First babies tend to be slightly smaller than siblings born later.
- Multiple pregnancies: Twins average about 5 pounds 5 ounces each, considerably less than singletons.
Maternal age plays a role too. Very young mothers (under 17) and those over 40 have higher rates of low birth weight babies, though the reasons differ. In younger mothers it’s often related to their own bodies still growing, while in older mothers it’s more often linked to placental function.
Weight Loss in the First Few Days
Almost all newborns lose weight before they start gaining it. This catches many new parents off guard, but it’s completely normal. Babies are born with extra fluid that they shed in the first few days of life, and they’re still learning to feed efficiently.
Most babies begin regaining weight between 3 and 5 days after birth. About 80 percent are back to their birth weight by 2 weeks of age. A weight loss of up to 7 or 8 percent is typical and not a concern. If a baby loses more than 10 percent of their birth weight, that signals a need for closer evaluation, usually of feeding technique and milk supply.
After that initial dip, healthy newborns gain about 5 to 7 ounces per week for the first several months. By 5 months, most babies have doubled their birth weight, and by their first birthday, they’ve roughly tripled it.
When Birth Weight Falls Outside the Typical Range
Babies under 5 pounds 8 ounces (2,500 grams) are considered low birth weight. Within that category, babies under 3 pounds 5 ounces (1,500 grams) are classified as very low birth weight. These smaller babies often need extra support with temperature regulation, feeding, and sometimes breathing, depending on whether their size is related to premature birth or growth restriction during pregnancy.
On the other end, babies above 8 pounds 13 ounces (4,000 grams) are considered large. This is more common in pregnancies affected by gestational diabetes, in parents who are naturally larger, and in pregnancies that go past the due date. Larger babies sometimes have more complicated deliveries but generally do well after birth.
Regardless of where your baby falls on the scale, what matters most is their growth trajectory over time. A baby born at the 15th percentile who stays near the 15th percentile is growing exactly as expected. Pediatricians watch for significant jumps or drops between percentile lines, not for a specific target number.