The average newborn weighs about 8 pounds (3,600 grams), though healthy babies routinely arrive anywhere from 5.5 to 8.8 pounds. That range depends on a mix of genetics, gestational age, and the birth parent’s health during pregnancy. Here’s what to expect from birth through the first year.
Average Birth Weight and Normal Range
Most full-term babies born between 37 and 40 weeks weigh between 5.5 and 8.8 pounds. The 8-pound average is just that: an average. A 6-pound baby and a 9-pound baby can both be perfectly healthy. Boys tend to weigh slightly more than girls at birth, and first babies often weigh a little less than siblings born later.
Babies weighing under 5.5 pounds (2,500 grams) at birth are classified as low birth weight by the World Health Organization. On the other end, a baby weighing more than 8 pounds 13 ounces (4,000 grams) is considered unusually large, a condition called macrosomia. Both extremes can require extra monitoring, but neither is automatically a sign of a serious problem.
What Affects Birth Weight
Genetics plays the most obvious role. Parents who are taller or larger tend to have bigger babies, and smaller parents tend to have smaller ones. But several other factors shift the number on the scale:
- Gestational age: Babies born even a few weeks early weigh less than those who reach full term. Each week in the womb adds meaningful weight, especially in the final month.
- Multiple pregnancies: Twins and triplets almost always weigh less individually than singletons because they share space and nutrients.
- Maternal diabetes: High blood sugar during pregnancy causes the baby to store extra fat and grow larger than average.
- Weight gain during pregnancy: Gaining significantly more than recommended is linked to larger babies.
- Placenta function: The placenta delivers oxygen and nutrients. If it isn’t working well, the baby may not grow as expected.
- Substance use or certain medications: Smoking, alcohol, and some prescription drugs during pregnancy can restrict fetal growth.
- Infections during pregnancy: Certain infections can interfere with growth before birth.
Weight Loss Right After Birth
Nearly all newborns lose weight in the first few days of life. This is normal. Babies are born with extra fluid, and they lose it before milk supply is fully established (whether breastfeeding or formula feeding). A loss of up to about 7% of birth weight is typical for most newborns. Weight loss that reaches 10% or more is a signal for closer evaluation, usually focusing on whether feeding is going well.
About 80% of babies regain their birth weight by two weeks of age. If your baby hasn’t returned to birth weight by that point, a pediatrician will likely want to assess feeding technique and overall intake. This isn’t unusual, and most of the time a small adjustment to feeding solves the issue.
How Fast Babies Gain Weight
Once feeding is established, newborns gain weight quickly. In the first few months, expect roughly 1 ounce (28 grams) per day. That’s close to half a pound a week, which is why babies seem to change so fast in those early weeks.
Around 4 months, the pace slows to about 20 grams a day. By 6 months, many babies are gaining 10 grams or less per day. This gradual slowdown is completely normal and reflects the shift from rapid early growth to a steadier trajectory. Babies who are more active, rolling and starting to move, naturally burn more of what they take in.
Doubling and Tripling Birth Weight
Two milestones pediatricians commonly reference: most babies double their birth weight by 6 months and triple it by their first birthday. So an 8-pound newborn would typically weigh around 16 pounds at 6 months and roughly 24 pounds at 12 months.
These are guidelines, not rigid targets. Some babies hit them a little early, others a little late. What matters more than any single number is a consistent growth curve. A baby who has been tracking along the 25th percentile and continues to do so is growing well, even if their actual weight is lower than the average. Sudden drops or jumps across percentile lines are what get a closer look.
Growth Charts and Percentiles
At every well-child visit, your baby’s weight gets plotted on a growth chart. The chart shows percentiles: if your baby is at the 40th percentile, that means 40% of babies the same age weigh less and 60% weigh more. There’s no “ideal” percentile. A baby at the 10th percentile is not less healthy than one at the 90th, as long as they’re growing consistently along their own curve.
Pediatricians typically use charts from the WHO for babies under 2 years old. These charts are based on breastfed infants as the standard, which matters because breastfed and formula-fed babies gain weight at slightly different rates. Breastfed babies often gain faster in the first 3 to 4 months and then slow down compared to formula-fed babies, who tend to gain more steadily through the second half of the first year.
When Weight Is a Concern
Birth weight outside the typical range doesn’t automatically mean something is wrong, but it does prompt extra attention. Low birth weight babies (under 5.5 pounds) may need help regulating body temperature and maintaining blood sugar in the first few days. Very small babies sometimes spend time in the NICU for feeding support.
Larger babies can occasionally have low blood sugar after delivery because their bodies were producing extra insulin in response to high glucose levels during pregnancy. They may also have a harder time during delivery itself. In both cases, the issues are usually short-lived and manageable.
After the newborn period, the concern shifts from absolute weight to the pattern of weight gain. A baby who stops gaining or loses weight after the first week needs evaluation. So does a baby whose weight percentile drops sharply over two or more visits. On the flip side, very rapid weight gain in infancy is generally not a concern the way it would be in older children, since babies are supposed to grow fast.