The average weight of a full-term newborn is about 7 pounds (3.2 kg). Most healthy babies born between 37 and 41 weeks of gestation fall somewhere between 5.5 and 10 pounds, though the exact number depends on several factors including gestational age, sex, and maternal health.
How Gestational Age Affects Birth Weight
Every week of pregnancy adds meaningful weight. A baby born at 37 weeks (the earliest point still considered full-term) typically weighs a full pound less than one born at 40 weeks. Canadian population data on singleton births breaks this down clearly:
- 37 weeks: Boys average 6 lbs 13 oz (3,091 g), girls average 6 lbs 9 oz (2,981 g)
- 38 weeks: Boys average 7 lbs 5 oz (3,306 g), girls average 7 lbs (3,181 g)
- 39 weeks: Boys average 7 lbs 11 oz (3,489 g), girls average 7 lbs 6 oz (3,350 g)
- 40 weeks: Boys average 8 lbs (3,638 g), girls average 7 lbs 11 oz (3,486 g)
Boys consistently weigh about 3 to 5 ounces more than girls at every gestational age. This gap is small but reliable across large populations.
When Birth Weight Is Considered Too Low or Too High
Babies weighing less than 5 pounds, 8 ounces (2,500 g) at birth are classified as low birth weight. Those under 3 pounds, 5 ounces (1,500 g) fall into the very low birth weight category. These thresholds matter because smaller babies face higher risks of breathing difficulties, feeding challenges, and trouble maintaining body temperature in the first days of life.
On the other end, a baby estimated to weigh more than 8 pounds, 13 ounces (4,000 g) is considered unusually large, a condition sometimes called macrosomia. Health risks increase significantly when weight exceeds 9 pounds, 15 ounces (4,500 g), including a higher chance of complicated delivery for both the mother and baby.
What Influences a Baby’s Birth Weight
Maternal body weight plays a substantial role. Babies born to mothers with a BMI under 18.5 (considered underweight) are nearly seven times more likely to have low birth weight compared to babies born to mothers with a BMI above 25. This is one of the strongest individual predictors researchers have identified.
Other factors that push birth weight up or down include:
- Nutrition during pregnancy: Consistent calorie and nutrient intake supports fetal growth, particularly in the third trimester when babies gain the most weight.
- Number of previous pregnancies: First babies tend to weigh slightly less than subsequent ones, though having a very high number of pregnancies (five or more) is associated with increased risk of low birth weight.
- Multiple pregnancies: Twins and triplets almost always weigh less than singletons because they share space and resources in the uterus.
- Gestational diabetes: Elevated blood sugar during pregnancy can lead to larger babies, which is one reason glucose screening is standard in prenatal care.
- Smoking: Tobacco use during pregnancy restricts blood flow to the placenta and consistently lowers birth weight.
Genetics also matters. Taller parents tend to have larger babies, and birth weight patterns often run in families.
Weight Loss in the First Week Is Normal
Nearly every newborn loses weight in the first few days after birth, which catches many new parents off guard. This drop is mostly fluid loss, not a sign of feeding problems. Healthy full-term babies typically lose up to 7% of their birth weight before they start gaining again. Some babies lose as much as 10%, though losses beyond that point usually prompt closer monitoring.
Most babies regain their birth weight by day 10. After that, weight gain picks up quickly. During the first three months, babies gain roughly an ounce per day on average, which works out to about half a pound per week. By the time a baby is two weeks old, the scale should be trending upward at each check.
How Birth Weight Is Tracked
Pediatricians plot your baby’s weight on a growth chart at every visit, comparing it to percentiles based on age and sex. A baby in the 50th percentile weighs more than half of babies the same age and less than the other half. What matters most isn’t the specific percentile but the trajectory over time. A baby who consistently tracks along the 25th percentile is growing normally, while a baby who drops from the 75th to the 25th percentile over a few visits may need evaluation.
Growth charts used in the U.S. come from the World Health Organization for children under two. These charts reflect how breastfed and formula-fed babies grow and provide a realistic benchmark. Your baby’s weight at birth is just the starting point on a curve that your pediatrician will follow for years.