The average full-term baby weighs about 7 pounds (3.2 kg) at birth. Full-term means born between 37 and 41 weeks of pregnancy. But that single number hides quite a bit of variation. A healthy birth weight can range from roughly 5.5 pounds to nearly 10 pounds, depending on gestational age, sex, genetics, and other factors.
How Birth Weight Changes by Week and Sex
Every extra week in the womb adds meaningful weight. A baby born at 37 weeks (the earliest point still considered full-term) typically weighs about 6 pounds 12 ounces, while a baby born at 40 weeks averages closer to 8 pounds. That’s nearly a full pound and a half of difference just from three more weeks of growth.
Boys tend to weigh slightly more than girls at every gestational age. At 40 weeks, the median weight for a boy is about 7 pounds 15 ounces (3,613 g), compared to 7 pounds 10 ounces (3,470 g) for a girl. Here’s how the averages break down by week:
- 37 weeks: Boys ~6 lb 13 oz, Girls ~6 lb 9 oz
- 38 weeks: Boys ~7 lb 4 oz, Girls ~7 lb 0 oz
- 39 weeks: Boys ~7 lb 10 oz, Girls ~7 lb 6 oz
- 40 weeks: Boys ~7 lb 15 oz, Girls ~7 lb 10 oz
These are median figures, meaning half of babies weigh more and half weigh less. The normal range is wide. At 40 weeks, a boy anywhere from about 6 pounds 4 ounces up to 9 pounds 15 ounces falls within the 3rd to 97th percentile, which is the range most doctors consider typical.
What Influences Birth Weight
Genetics plays the biggest role. Taller, larger parents tend to have bigger babies, and birth weight patterns often run in families. But several other factors shift the number up or down.
Maternal weight gain during pregnancy has a direct, measurable effect. For every additional kilogram (about 2.2 pounds) a mother gains during pregnancy, her baby’s birth weight increases by roughly 7 grams. That sounds tiny, but it adds up. Women who gain more than 53 pounds during pregnancy have babies that are, on average, about 5 ounces heavier than babies born to women who gain 18 to 22 pounds. They’re also more than twice as likely to deliver a baby weighing over 8 pounds 13 ounces.
Other factors that tend to increase birth weight include gestational diabetes, having had previous pregnancies, and going past your due date. Factors linked to lower birth weight include smoking, high blood pressure, carrying twins or multiples, and being a first-time parent.
When Birth Weight Is Considered Too Low
The World Health Organization defines low birth weight as anything under 5 pounds 8 ounces (2,500 grams). In the United States, about 8.6% of babies born in 2023 fell into this category. Very low birth weight, under 3 pounds 5 ounces (1,500 grams), is much rarer at 1.4% of births.
Low birth weight most commonly results from premature birth. A baby born at 34 weeks may weigh under 5 pounds simply because they haven’t had enough time to grow, not because anything else is wrong. In other cases, a baby can be full-term but small due to restricted growth in the womb, sometimes caused by placental problems or maternal health conditions. Low-birth-weight babies are more closely monitored for feeding difficulties, temperature regulation, and blood sugar levels in the first days of life.
When Birth Weight Is Considered Too High
A baby weighing more than 8 pounds 13 ounces (4,000 g) is considered large for gestational age. The health risks increase more sharply above 9 pounds 15 ounces (4,500 g), a condition sometimes called macrosomia. About 8% of deliveries in the U.S. involve a baby over 8 pounds 13 ounces.
Larger babies carry a higher risk of birth injuries during vaginal delivery, particularly shoulder injuries if the baby’s shoulders become stuck during birth. For the mother, delivering a very large baby increases the chance of tearing and, for women with a prior cesarean scar, raises the risk of uterine rupture during labor. After birth, very large babies are more likely to have low blood sugar and breathing difficulties in the first hours. They also carry a modestly higher risk of obesity later in childhood.
Normal Weight Loss After Birth
Almost every newborn loses weight in the first few days. This is completely normal and expected. Babies are born with extra fluid reserves specifically designed to carry them through those early days before a mother’s full milk supply comes in, which typically happens around 72 hours after delivery.
A healthy full-term baby will lose up to 7% of their birth weight before the trend reverses. For a 7-pound baby, that’s about half a pound. Weight loss beyond 10% is a signal that feeding may need to be evaluated. Most babies regain their birth weight within 10 to 14 days. If your baby hasn’t returned to their birth weight by the two-week mark, your pediatrician will likely want to assess whether feeding is going well and whether there’s an underlying issue slowing growth.
After that initial dip, healthy newborns typically gain about 5 to 7 ounces per week for the first several months, roughly doubling their birth weight by around 5 months of age.