The average newborn weighs about 7.6 pounds (3,400 grams). Most full-term babies fall somewhere between 5.5 and 10 pounds, with the majority clustering close to that average. But birth weight varies quite a bit depending on gestational age, genetics, and the mother’s health during pregnancy.
What Counts as Normal Birth Weight
A healthy full-term baby, born between 39 and 41 weeks, typically weighs between 6 and 9 pounds. Boys tend to weigh slightly more than girls at birth, though the difference is usually just a few ounces. First-born babies also tend to be a bit lighter than their younger siblings.
Babies born below 5 pounds 8 ounces (2,500 grams) are classified as low birth weight. Those under 3 pounds 5 ounces (1,500 grams) are considered very low birth weight. On the other end of the spectrum, babies weighing more than 8 pounds 13 ounces (4,000 grams) are considered larger than average, and those above roughly 10 pounds (4,500 grams) fall into a category called macrosomia. About 90% of full-term babies land between these extremes.
Why Babies Lose Weight After Birth
Nearly all newborns lose weight in their first few days of life. This is normal and expected. Babies are born with extra fluid, and they shed it through urine and stool before feeding is fully established. Most lose between 5% and 7% of their birth weight, and breastfed babies often lose a bit more than formula-fed babies.
Delivery method matters too. Research published in Pediatrics found that more than 25% of babies born by cesarean section lost 10% or more of their birth weight within 72 hours. This is partly because mothers who deliver surgically sometimes experience a slight delay in milk production. Most pediatricians consider weight loss up to about 10% acceptable in the first few days, but anything beyond that gets a closer look.
Babies typically regain their birth weight by 10 to 14 days old. If your baby hasn’t bounced back by the two-week mark, the pediatrician will likely want to evaluate feeding.
What Affects Birth Weight
Genetics plays the biggest role. Taller, larger parents tend to have bigger babies. But several other factors shift birth weight up or down in meaningful ways.
- Gestational age: Every week of pregnancy adds weight. A baby born at 37 weeks will almost always weigh less than one born at 40 weeks, even though both are technically considered term.
- Maternal nutrition: Long-term maternal malnutrition is one of the strongest predictors of low birth weight, according to the World Health Organization. On the flip side, excessive weight gain during pregnancy is linked to larger babies.
- Gestational diabetes: When blood sugar runs high during pregnancy, the baby receives more glucose than it needs and stores the excess as fat. This is one of the most common causes of babies weighing over 9 or 10 pounds.
- Smoking: Smoking restricts blood flow to the placenta, reducing the oxygen and nutrients the baby receives. It consistently lowers birth weight by several ounces on average.
- Multiple births: Twins weigh less than singletons at birth, typically arriving between 5 and 6 pounds each. They share space and placental resources, which limits individual growth, especially in the third trimester.
Weight Gain in the First Month
Once a newborn regains birth weight, growth accelerates. In the first few months of life, babies gain about 1 ounce (28 grams) per day, according to the Mayo Clinic. That works out to roughly half a pound per week, or about 2 pounds per month. Most babies double their birth weight by around 4 to 5 months and triple it by their first birthday.
Breastfed and formula-fed babies grow at slightly different rates. Breastfed babies often gain weight faster in the first few months, then slow down relative to formula-fed babies in the second half of the first year. Both patterns are normal. Pediatricians track growth on percentile charts over time, and what matters most is that your baby follows a consistent curve rather than hitting one specific number.
When Birth Weight Is a Concern
Low birth weight is more common in premature babies, but it can happen at full term too. Babies born under 5 pounds 8 ounces sometimes need extra support with feeding and temperature regulation, since they have less body fat to keep themselves warm. Very small babies may spend time in the NICU, though many catch up in growth within the first year or two.
Large babies present their own set of challenges. Babies over 9 or 10 pounds are more likely to need assisted delivery, and they carry a higher risk of low blood sugar in the hours after birth because their bodies have been processing extra glucose from the mother. These babies are monitored closely but rarely face long-term complications from their size alone.
In both cases, birth weight is a snapshot. What your baby weighs on day one matters less than how they grow in the weeks and months that follow. A baby born at the 15th percentile who stays on that curve is doing exactly what they should be.