The average full-term baby weighs about 7 pounds (3.2 kg) at birth. Most healthy newborns fall somewhere between 5.5 and 10 pounds, with the exact number shaped by genetics, the mother’s health during pregnancy, and whether the baby arrived early or on time.
What Counts as a Healthy Birth Weight
A full-term baby, born between 37 and 41 weeks of pregnancy, typically lands close to that 7-pound average. But there’s a wide window of normal. Babies under 5 pounds 8 ounces (2,500 grams) are classified as low birth weight, which can signal that the baby needs extra monitoring or medical support in the first days of life. On the other end, babies over about 8 pounds 13 ounces (4,000 grams) are considered large for gestational age, a condition called macrosomia. At the more extreme threshold of 9 pounds 15 ounces (4,500 grams), the risk of delivery complications rises more sharply.
Boys tend to weigh slightly more than girls at birth. In one large U.S. study, the average male singleton weighed about 7 pounds 8 ounces (3,410 grams), while the average female singleton came in around 7 pounds 5 ounces (3,320 grams).
Twins Weigh Considerably Less
If you’re expecting multiples, the numbers look quite different. Twins share space and resources in the womb, and they also tend to arrive earlier. Male twins average about 5 pounds 10 ounces (2,570 grams) at birth, while female twins average around 5 pounds 6 ounces (2,440 grams). That’s roughly a pound and a half to two pounds lighter than singletons. Despite the lower starting weight, most healthy twins follow their own growth curves and catch up over the first year or two.
Why Babies Lose Weight Right After Birth
Nearly every newborn loses weight in the first few days. This catches many new parents off guard, but it’s normal. Babies are born with extra fluid, and they shed it before feeding is fully established. Breastfed newborns tend to lose a bit more than formula-fed babies because breast milk takes a few days to come in fully.
Research tracking breastfed newborns found they lose about 4.5% of their birth weight by 24 hours, around 7.6% by 48 hours, and roughly 8% by 72 hours. A loss of up to about 7% is generally considered unremarkable. Once it exceeds 10% of birth weight, the risk of dehydration and other complications increases, and your baby’s care team will likely step in with a feeding plan. Most babies regain their birth weight within 10 to 14 days.
How Fast Babies Gain Weight in the First Year
After that initial dip, healthy babies gain weight quickly. In the first few months, the typical infant puts on about 1 ounce (28 grams) per day. That pace slows to roughly 20 grams a day around the four-month mark, and by six months, many babies are gaining 10 grams or less per day. This gradual slowdown is completely expected and reflects the natural shift from rapid early growth to a more moderate pace.
A useful rule of thumb: most babies double their birth weight by four to five months and triple it by their first birthday. For a baby born at 7 pounds, that means reaching roughly 14 pounds around month four or five, and about 21 pounds by age one. Between 10 and 12 months, the average infant gains about 13 ounces per month.
Keep in mind these are averages. A baby who started small will likely stay on a lower growth curve, and a bigger baby may stay above average. What matters most is a consistent pattern of gain, not hitting one specific number. Pediatricians track this on standardized growth charts developed by the World Health Organization, plotting your baby’s weight at each visit to make sure the overall trend stays steady.
What Influences Birth Weight
Several factors tilt a baby’s weight up or down before they’re even born. Some are within a parent’s control, and some aren’t.
- Genetics: Taller and larger parents tend to have bigger babies. This is one of the strongest predictors and isn’t something you can change.
- Gestational age: Babies born before 37 weeks haven’t had the final stretch of rapid weight gain that happens in the last weeks of pregnancy. Even a week or two early can make a noticeable difference on the scale.
- Maternal nutrition: Inadequate food intake during pregnancy is linked to a higher chance of low birth weight. Eating enough calories and nutrients, particularly protein, iron, and folate, supports healthy fetal growth.
- Tobacco smoke exposure: Exposure to secondhand smoke during pregnancy carries nearly a fourfold increase in the risk of delivering a low-birth-weight baby. Direct smoking raises the risk even further. This is one of the most impactful modifiable risk factors.
- Gestational diabetes: When blood sugar runs high during pregnancy, the baby receives extra glucose and stores it as fat. This is one of the most common reasons babies tip into the macrosomia range.
- Birth order: First babies tend to be slightly smaller than later siblings, though the difference is usually modest.
When Weight Becomes a Concern
In the newborn period, losing more than 10% of birth weight or failing to regain birth weight within two weeks can signal feeding difficulties or other issues that need attention. For breastfed babies, this often means adjusting latch technique or increasing feeding frequency rather than anything more complicated.
In the months that follow, the pattern of weight gain matters more than any single measurement. A baby who drops from one growth curve to a significantly lower one over two or three visits may need evaluation, while a baby who’s consistently in the 15th percentile is likely just on the smaller side of normal. Similarly, a sudden jump upward can sometimes reflect overfeeding with formula but is rarely a concern in breastfed infants.
Premature babies and twins often follow adjusted growth expectations based on their due date rather than their actual birth date. A baby born six weeks early, for example, would be compared against growth standards for an infant six weeks younger until around age two, when most preemies have caught up to their full-term peers.