The average newborn weighs between 5 pounds 8 ounces and 8 pounds 13 ounces (2,500 to 4,000 grams), with most full-term babies landing right around 7.5 pounds. That range shifts depending on gestational age, sex, genetics, and whether the baby is a singleton or a multiple. Here’s what those numbers actually mean and what to expect in the first days after birth.
Average Weight by Sex and Gestational Age
A baby’s weight at birth depends heavily on how many weeks of pregnancy have passed. A baby born at 34 weeks weighs considerably less than one born at 40 weeks, and boys tend to run slightly heavier than girls at every stage.
For singleton births, here’s how average weights break down:
- 34 weeks: Boys average 5 lbs 3 oz (2,361 g); girls average 5 lbs 0 oz (2,266 g)
- 37 weeks: Boys average 6 lbs 13 oz (3,091 g); girls average 6 lbs 9 oz (2,981 g)
- 40 weeks: Boys average 8 lbs 0 oz (3,638 g); girls average 7 lbs 11 oz (3,486 g)
That gap between 34 and 40 weeks is striking. A baby gains roughly a half pound per week during the final stretch of pregnancy, which is why even a few weeks of earlier delivery can make a noticeable difference on the scale.
What Counts as Low or High Birth Weight
Babies born weighing less than 5 pounds 8 ounces (2,500 grams) are classified as low birth weight. These babies often need extra monitoring and sometimes specialized care in a neonatal unit, depending on how far below the threshold they fall and whether they were born early.
On the other end, babies weighing 8 pounds 13 ounces (4,000 grams) or more are considered large for gestational age. The medical term is macrosomia. The American College of Obstetricians and Gynecologists breaks this into categories: 4,000 to 4,499 grams, 4,500 to 4,999 grams, and 5,000 grams or above, with the risk of complications during delivery rising at each tier. Macrosomia is more common in pregnancies involving gestational diabetes, post-due-date deliveries, and parents who are naturally larger.
Why Some Babies Weigh More or Less
Gestational age is the single biggest factor. Premature birth (before 37 weeks) is the strongest predictor of low birth weight by a wide margin. But several maternal health factors also play a role.
Pregnancy-related high blood pressure roughly doubles the odds of a smaller baby. A mother with a very low body mass index before pregnancy (under 18.5) faces significantly higher odds of delivering a low-birth-weight infant as well. Nutrition during pregnancy, smoking, and chronic health conditions like diabetes all shift the number in one direction or the other. Genetics matter too: taller parents with larger frames tend to have bigger babies, and first-born children often weigh slightly less than their siblings.
Twins and other multiples almost always weigh less than singletons. They share space and nutritional resources in the womb, and they’re frequently delivered earlier. It’s completely normal for twins to arrive in the 4 to 6 pound range.
Weight Loss in the First Week
Nearly every newborn loses weight in the first few days of life, and this catches many new parents off guard. Healthy newborns typically lose 7 to 10 percent of their birth weight during the first week. For a 7.5-pound baby, that’s roughly half a pound to three-quarters of a pound. This happens because babies are born with extra fluid and because it takes a few days for breastfeeding to fully establish or for formula feeding routines to settle in.
Most babies regain their birth weight by two weeks of age. After that initial dip, expect a gain of about 1 ounce (30 grams) per day. Your baby’s pediatrician will track weight at each early visit, so you’ll have a clear picture of whether things are on course. A baby who hasn’t returned to birth weight by the two-week mark typically gets closer follow-up to make sure feeding is going well.
What the Number on the Scale Actually Tells You
Birth weight is one of the first health data points your baby gets, and it serves as a baseline for tracking growth. But a single number at birth doesn’t predict your child’s long-term size or health. A 6-pound baby born at 37 weeks is in a completely different situation than a 6-pound baby born at 41 weeks, even though the scale reads the same.
What matters more than the exact weight is the trajectory afterward. Pediatricians plot your baby’s weight on a growth curve and look for consistent, steady gains over weeks and months. A baby who starts at the 25th percentile and stays near the 25th percentile is growing perfectly well. The concern arises when a baby’s growth line starts crossing percentiles in either direction, which can signal feeding difficulties or other issues worth investigating.
If your baby falls within that broad 5.5 to 8.8 pound range at birth and is gaining steadily in the weeks that follow, the number itself is far less important than the pattern it’s part of.