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 exact number depending on gestational age, sex, genetics, and the health of the pregnant parent. Here’s what shapes that number and what the different weight ranges mean.
Average Birth Weight by Week
Babies gain weight rapidly in the final weeks of pregnancy, so even a week or two of extra time in the womb makes a noticeable difference. At 37 weeks (the earliest point considered “early term”), boys average about 6 pounds 13 ounces and girls about 6 pounds 9 ounces. By 40 weeks, boys average roughly 8 pounds and girls about 7 pounds 11 ounces.
That weekly gain works out to roughly half a pound per week in the late third trimester. This is one reason doctors pay close attention to gestational age when evaluating a newborn’s weight. A 6-pound baby born at 37 weeks is in a very different situation than a 6-pound baby born at 40 weeks.
Boys vs. Girls
Male newborns consistently weigh more than female newborns by about 3 to 5 ounces on average. In large population studies, boys have averaged around 7 pounds 12 ounces while girls have averaged closer to 7 pounds 7 ounces. The gap is small but remarkably consistent across countries and time periods. It shows up at every gestational age from 37 weeks onward.
Twins and Multiples
Twins are substantially smaller than singletons at birth. The average twin weighs about 5 pounds 5 ounces, roughly 28% less than the average singleton. This isn’t because something went wrong. Twins share space and nutrients, and they’re more likely to arrive early. Most twins are born between 35 and 37 weeks, which accounts for much of the weight difference. Triplets and higher-order multiples tend to be smaller still, often arriving before 35 weeks and frequently weighing under 4 pounds each.
What Counts as Low or High Birth Weight
Doctors use specific cutoffs to flag babies who may need extra monitoring:
- Low birth weight: under 5 pounds 8 ounces (2,500 grams). About 8.6% of babies born in the U.S. in 2023 fell into this category.
- Very low birth weight: under 3 pounds 5 ounces (1,500 grams). These babies almost always need time in the NICU.
- High birth weight (macrosomia): over 8 pounds 13 ounces (4,000 grams). Risks to both parent and baby increase as weight climbs above this threshold, with the highest concern for babies over 9 pounds 15 ounces (4,500 grams).
These aren’t just academic categories. Low birth weight babies are more likely to have difficulty maintaining body temperature and feeding in those first days. Very large babies raise the chance of birth complications, including shoulder injuries during delivery and the need for a cesarean section.
What Influences Birth Weight
Genetics set the baseline. If both parents are tall and large-framed, the baby is more likely to be on the heavier end, and the reverse is also true. But several other factors push weight up or down from that genetic starting point.
Gestational diabetes is one of the strongest drivers of high birth weight. When blood sugar runs high during pregnancy, the baby receives more glucose than it needs and stores the excess as fat. Gaining too much weight during pregnancy has a similar, though less dramatic, effect. On the other side, conditions like preeclampsia can restrict blood flow to the placenta and lead to a smaller baby.
Nutrition matters in a practical way. Consistently getting too few calories or key nutrients during pregnancy is linked to lower birth weight, while adequate nutrition supports healthy fetal growth. Smoking during pregnancy reliably reduces birth weight, often by half a pound or more. First-born babies also tend to weigh slightly less than later siblings.
Weight Loss in the First Few Days
Nearly all newborns lose weight after birth, which catches many new parents off guard. Healthy full-term babies typically lose up to 7% of their birth weight in the first three to four days. For a 7.5-pound baby, that’s about half a pound. This happens because babies shed extra fluid they carried from the womb and because feeding is just getting established.
Most babies regain their birth weight by about day 10. If a baby loses more than 10% of birth weight or hasn’t bounced back by two weeks, pediatricians investigate whether feeding is going well and whether there’s an underlying issue. Breastfed babies sometimes lose a bit more than formula-fed babies in those early days, simply because milk supply takes a couple of days to fully come in.
How Birth Weight Is Tracked Over Time
Your baby’s birth weight is plotted on a growth chart that compares them to other babies of the same sex and gestational age. The result is a percentile. A baby in the 50th percentile weighs more than half of all babies and less than the other half. A baby in the 10th percentile is lighter than 90% of peers but still within the normal range.
What matters most isn’t the single number at birth but the trajectory afterward. A baby born at the 25th percentile who stays near the 25th percentile over the coming months is growing exactly as expected. A baby who drops sharply from the 50th to the 10th percentile in the first few weeks warrants a closer look. Pediatricians track these curves at every well-child visit during the first year, when growth is fastest and most informative.