The average newborn weighs about 3.3 kilograms, or roughly 7 pounds 4 ounces. Most full-term babies fall somewhere between 2.5 and 4 kilograms (5 pounds 8 ounces to 8 pounds 13 ounces), and where your baby lands in that range depends on gestational age, sex, and maternal health factors.
How Gestational Age Affects Birth Weight
Every week in the womb adds meaningful weight. A baby born at 37 weeks, the earliest point still considered full-term, weighs noticeably less than one born at 40 weeks. Population data from Canada illustrates the progression clearly:
- 37 weeks: about 3,000 to 3,100 g (6 lb 10 oz to 6 lb 13 oz)
- 38 weeks: about 3,180 to 3,300 g (7 lb 0 oz to 7 lb 4 oz)
- 39 weeks: about 3,350 to 3,490 g (7 lb 6 oz to 7 lb 11 oz)
- 40 weeks: about 3,490 to 3,640 g (7 lb 11 oz to 8 lb 0 oz)
That means a baby born three weeks early could weigh a full pound less than one born at the due date, even though both are considered full-term. This is one reason your provider tracks gestational age so closely.
Boys vs. Girls
Male newborns tend to be slightly heavier. On average, boys weigh about 3.34 kg (7 lb 6 oz) while girls weigh about 3.26 kg (7 lb 3 oz). That gap of roughly 85 grams, or 3 ounces, is consistent across the gestational age chart. At 40 weeks, for instance, the average male singleton weighs 3,638 g compared to 3,486 g for a female singleton, a difference of about 150 grams (5 ounces).
When Birth Weight Is Considered Low
A baby born weighing less than 2,500 grams (5 pounds 8 ounces) is classified as low birth weight. Below 1,500 grams (3 pounds 5 ounces) is considered very low birth weight. These thresholds matter because smaller babies are more likely to need extra medical support for temperature regulation, feeding, and breathing. Prematurity is the most common reason for low birth weight, but it can also happen in full-term babies when growth is restricted during pregnancy.
When Birth Weight Is Considered High
On the other end, a baby weighing 4,000 grams (8 pounds 13 ounces) or more is classified as having macrosomia. Clinicians break this into categories: 4,000 to 4,499 g, 4,500 to 4,999 g, and 5,000 g or above. The risk of delivery complications rises with each tier. Macrosomia is more common when the mother has diabetes or gestational diabetes, or when pregnancy extends past 40 weeks.
Maternal Health and Birth Weight
A mother’s weight and metabolic health play a significant role. Gestational diabetes on its own tends to increase birth length, but when it combines with higher maternal BMI, the effect on birth weight becomes much more pronounced. Research published in Frontiers in Pediatrics found that the increase in birth weight associated with higher maternal BMI was significantly stronger in pregnancies with gestational diabetes than without it. In practical terms, a mother with both a higher BMI and gestational diabetes is more likely to deliver a larger baby than either factor alone would predict.
Other factors that influence birth weight include genetics and ethnicity, whether the baby is a singleton or part of a multiple pregnancy, the mother’s nutrition during pregnancy, and smoking. Twins and triplets are almost always smaller than singletons born at the same gestational age.
Normal Weight Loss After Birth
Almost every newborn loses weight in the first few days of life. This catches many new parents off guard, but it’s completely expected. Babies are born with extra fluid, and they’re still learning to feed efficiently. A loss of up to 7% of birth weight is normal for full-term infants. For a 3.4 kg baby, that amounts to about 240 grams, or roughly half a pound.
Most healthy newborns regain their birth weight within 10 to 14 days. Pediatricians typically check weight at the first office visit (usually 3 to 5 days after birth) and again at the two-week mark specifically to track this recovery. If a baby hasn’t returned to birth weight by two weeks, or loses more than 10% of birth weight at any point, your pediatrician will want to evaluate feeding and look into possible causes.
What Birth Weight Tells You
Birth weight is one of the first health markers your baby receives, but it’s just a snapshot. A baby born at 6 pounds 10 ounces who feeds well and gains steadily is perfectly healthy. What matters more than the number on the scale at delivery is the trajectory afterward: consistent weight gain along a growth curve over the first weeks and months. Your baby’s percentile at birth is less important than whether they stay on a steady growth path over time.