The normal weight for a full-term newborn is between about 5 pounds 8 ounces and 8 pounds 13 ounces (2,500 to 4,000 grams). The average sits right around 7 pounds 5 ounces (3,300 grams), though healthy babies routinely fall above or below that number. Where your baby lands depends on gestational age, sex, genetics, and several maternal health factors.
The Typical Range at Birth
A baby born at 40 weeks will generally weigh more than one born at 37 weeks, even though both are considered full-term. Population data from Canada illustrates this clearly: at 40 weeks, the median weight for boys is about 7 pounds 15 ounces (3,613 grams), while girls come in slightly lighter at 7 pounds 10 ounces (3,470 grams). At 37 weeks, those medians drop to roughly 6 pounds 12 ounces for boys and 6 pounds 8 ounces for girls.
The 10th-to-90th percentile span, which covers the vast majority of healthy newborns, is wide. A 40-week boy anywhere from about 6 pounds 12 ounces to 9 pounds 4 ounces falls within that range. For girls at 40 weeks, the equivalent window runs from about 6 pounds 8 ounces to 8 pounds 14 ounces. Pediatricians in the United States plot these numbers against WHO growth standards, which are the recommended charts for children from birth to age two.
Boys Versus Girls
Male newborns consistently weigh a bit more than females at every gestational age. The gap is modest, usually 100 to 150 grams (3 to 5 ounces), but it’s present across populations. This difference continues through infancy and is reflected in the separate growth chart curves your pediatrician uses for boys and girls.
When Birth Weight Is Considered Low
Any baby weighing less than 5 pounds 8 ounces (2,500 grams) at birth is classified as low birth weight, regardless of how many weeks the pregnancy lasted. Within that category, the thresholds get more specific. Below 3 pounds 5 ounces (1,500 grams) is very low birth weight, and below 2 pounds 3 ounces (1,000 grams) is extremely low birth weight. Lower weight categories carry higher risks for breathing problems, difficulty regulating temperature, and feeding challenges in the first days and weeks.
Premature birth is the most common reason a baby is small, but it’s not the only one. Substance use during pregnancy, placental problems, and certain infections can all restrict fetal growth even when a baby is carried to term.
When Birth Weight Is Considered High
A baby weighing more than 8 pounds 13 ounces (4,000 grams) is on the larger end, and the term macrosomia applies once birth weight reaches that threshold. Babies above 9 pounds 15 ounces (4,500 grams) carry a higher risk of delivery complications for both baby and mother, including shoulder injuries during birth and the increased likelihood of a cesarean delivery.
The most well-established driver of high birth weight is diabetes during pregnancy, whether it existed before conception or developed as gestational diabetes. Excessive weight gain during pregnancy also increases the odds of delivering a larger baby. Genetics play a role too: taller parents and those who were larger at birth themselves tend to have bigger babies.
Birth Weight for Twins and Multiples
Twins are almost always lighter than singletons. Across multiple hospital populations, the average birth weight for twins falls between about 4 pounds 15 ounces and 5 pounds 6 ounces (2,245 to 2,444 grams), compared to 6 pounds 11 ounces to 7 pounds 2 ounces (3,041 to 3,222 grams) for singletons born at the same facilities. Part of this difference is explained by twins arriving earlier on average, but research shows that twin fetal growth slows compared to singletons after about 28 weeks, regardless of when delivery happens. So even twins carried to the same gestational age as a singleton will typically weigh less. Pediatricians account for this when assessing a twin’s growth, and a birth weight that would raise concerns for a singleton may be perfectly appropriate for a twin.
Normal Weight Loss After Birth
Nearly all newborns lose weight in the first few days of life. This is expected and happens because babies are born with extra fluid that they shed, and because milk intake in the first 48 to 72 hours is still small as feeding gets established. Most babies begin regaining weight between days three and five. About 80 percent are back to their birth weight by two weeks of age.
A weight loss of up to about 7 percent of birth weight is common. Losses beyond 10 percent signal that feeding needs a closer look. For a 7-pound baby, 10 percent is 11 ounces, so anything beyond that warrants a conversation with your pediatrician or midwife about whether the baby is latching effectively and getting enough milk.
Expected Weight Gain in the First Month
Once the initial dip resolves, healthy newborns gain weight at a surprisingly steady pace: roughly one ounce (28 grams) per day through the first month. That works out to about half a pound per week. By the one-month checkup, most babies weigh at least a pound or two more than their birth weight. Your pediatrician will plot this gain on a growth chart, and the trajectory matters more than any single number. A baby who tracks consistently along the 20th percentile is just as healthy as one tracking the 80th, as long as the curve stays steady.
What Influences Birth Weight
Gestational age is the single biggest factor. Each additional week in the womb adds roughly 150 to 200 grams (5 to 7 ounces) of weight between weeks 37 and 40. Beyond that, several maternal and genetic factors shift where a baby falls within the range:
- Parental size. Taller and larger parents tend to have bigger babies. This is partly genetic and partly reflects the uterine environment.
- Maternal diabetes. High blood sugar during pregnancy delivers more glucose to the baby, who stores it as fat. This is the most common medical cause of high birth weight.
- Pregnancy weight gain. Gaining significantly more than recommended increases the chance of a larger baby, while inadequate gain can contribute to a smaller one.
- Smoking and substance use. Both restrict blood flow to the placenta and are strongly associated with lower birth weight.
- Birth order. First babies tend to be slightly smaller than subsequent siblings.
- Multiple pregnancy. Twins and higher-order multiples share uterine space and nutrients, resulting in lower individual birth weights.
If your baby falls outside the “average” range but your pediatrician isn’t concerned, that’s usually because the baby’s weight makes sense in context. A 6-pound baby born at 37 weeks to a petite mother is on a completely different trajectory than a 6-pound baby born at 41 weeks after gestational diabetes. The number alone doesn’t tell the whole story.