An average full-term newborn weighs about 3.3 kg (7 lb 6 oz) for boys and about 3.2 kg (7 lb 1 oz) for girls, and the normal range for a full-term birth is 2.5 to 4.5 kg (5 lb 8 oz to 9 lb 15 oz). Those figures come from the World Health Organization growth standards, which are built from data collected across multiple countries and are widely used by pediatricians worldwide. If your baby falls anywhere in that 2.5 to 4.5 kg range, their birth weight is within what clinicians consider normal.
This article is general health information only and is not a substitute for professional medical advice, diagnosis, or treatment. If you have any concerns about your baby's weight or growth, speak with your pediatrician or healthcare provider.
Parents often switch between kilograms and pounds when reading growth charts, hospital paperwork, and apps, and the mental math is tedious. The converter below lets you flip any weight instantly. The weight-by-month chart and the detailed sections that follow use both units throughout.
Why birth weight varies from baby to baby
Birth weight is shaped by a mix of factors, and no two babies are alike. The most important single factor is gestational age: a baby born at 37 weeks will almost always weigh less than one born at 40 weeks, simply because they had less time to grow. Beyond gestational age, the following factors all play a role:
- Sex. Boys average slightly heavier than girls at birth, a pattern that persists through the first year and is reflected in the WHO growth charts used by the American Academy of Pediatrics and the CDC.
- Parental size and genetics. Taller parents and parents with larger frames tend to have heavier babies, though genetics is only part of the picture.
- Maternal nutrition and health. Good prenatal nutrition supports healthy fetal growth. Conditions such as gestational diabetes are associated with larger babies (macrosomia), while severe nutritional restriction or certain infections can restrict growth.
- Multiple pregnancy. Twins, triplets, and other multiples typically weigh less individually than singleton babies because they share resources in the womb.
- Smoking and substance use. Smoking during pregnancy is one of the strongest modifiable risk factors for low birth weight, according to the CDC.
Understanding these factors helps put a birth weight reading in context. A baby on the smaller side because of parental genetics is in a very different situation from a baby who is small due to restricted fetal growth, and your obstetric team will distinguish between the two during prenatal care.
Average baby weight by month: WHO 50th percentile chart
The table below shows approximate 50th-percentile weights from the WHO Child Growth Standards, which form the basis of growth charts recommended by both the American Academy of Pediatrics and the CDC for children from birth to two years. The 50th percentile means half of healthy babies weigh more and half weigh less at that age. Your baby does not need to match these numbers exactly; the goal is a steady upward curve on their own trajectory.
| Age | Boys (50th percentile) | Girls (50th percentile) |
|---|---|---|
| Birth | 3.3 kg (7 lb 6 oz) | 3.2 kg (7 lb 1 oz) |
| 1 month | 4.5 kg (9 lb 14 oz) | 4.2 kg (9 lb 4 oz) |
| 2 months | 5.6 kg (12 lb 5 oz) | 5.1 kg (11 lb 4 oz) |
| 3 months | 6.4 kg (14 lb 2 oz) | 5.8 kg (12 lb 12 oz) |
| 4 months | 7.0 kg (15 lb 7 oz) | 6.4 kg (14 lb 2 oz) |
| 6 months | 7.9 kg (17 lb 7 oz) | 7.3 kg (16 lb 1 oz) |
| 9 months | 8.9 kg (19 lb 10 oz) | 8.2 kg (18 lb 1 oz) |
| 12 months | 9.6 kg (21 lb 3 oz) | 8.9 kg (19 lb 10 oz) |
Notice how quickly babies gain weight in the first few months. From birth to 3 months, boys at the 50th percentile nearly double in weight, going from 3.3 kg to 6.4 kg. That rapid early growth gradually slows over the first year, which is normal and expected.
How to read percentiles honestly
Percentile lines on a growth chart are reference ranges, not grades. A baby at the 10th percentile for weight is not underweight; they are simply smaller than 90 percent of babies the same age, which can be entirely normal if that is their consistent pattern. What pediatricians look for is whether a baby is tracking steadily along their own curve.
The American Academy of Pediatrics considers it a reason for closer monitoring when a baby drops across two or more major percentile lines over time, for example falling from roughly the 50th percentile to the 10th over several months, or when weight gain slows or stops. A single measurement below a line is rarely a problem on its own. Growth is a pattern, not a snapshot.
The CDC recommends using the WHO growth charts for children under two years and the CDC growth charts for children two years and older. Both sets of charts show percentile lines at the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles. Babies in the bottom 3rd or top 97th percentile may warrant additional evaluation, but many are perfectly healthy.
Normal early weight loss and regain
It is completely normal for a newborn to lose weight in the first days of life. Babies are born with extra fluid in their tissues, and they lose it rapidly after birth. Before feeding is fully established, caloric intake is modest, and most newborns lose up to 10 percent of their birth weight by day 3 or 4.
A baby who weighed 3.5 kg at birth might drop to around 3.15 kg before starting to climb again. The turning point usually comes around day 3 to 5 when the mother's milk comes in or formula feeding is well underway. Most babies regain their birth weight by about two weeks of age. If a baby has not regained birth weight by two weeks, or if the weight loss exceeds 10 percent, the pediatrician will want to assess feeding and may recommend supplementation.
Doubling and tripling: the first-year milestones
Two widely cited benchmarks help parents understand whether growth is on track across the first year. Most babies roughly double their birth weight between 4 and 6 months, and most triple their birth weight by 12 months. The WHO 50th percentile figures in the chart above fit this pattern closely: a boy at the 50th percentile weighs 3.3 kg at birth, about 7.0 kg at 4 months (just over double), and 9.6 kg at 12 months (close to triple).
These milestones are approximations rather than hard deadlines. A baby who doubles birth weight at 5 months instead of 4 months, or triples it at 13 months instead of 12, is likely growing normally. Use the benchmarks as a rough orientation, not a strict schedule. Your pediatrician tracks weight at every well-child visit specifically because a series of measurements over time tells a far more complete story than any single weigh-in.
If you are also monitoring your baby's temperature and wondering what counts as a fever in an infant, see our related guide on normal temperature for babies, which covers rectal, axillary, and forehead readings for newborns and infants.
When to call the pediatrician
Most weight concerns in infancy are identified during routine well-child visits, but you do not need to wait for a scheduled appointment if something does not seem right. Contact your pediatrician if:
- Your newborn has not regained birth weight by about two weeks of age.
- Your baby loses more than 10 percent of birth weight at any point.
- Weight gain appears to stall for two or more consecutive weeks during the first year.
- Your baby drops significantly across percentile lines on the growth chart between visits.
- Your baby seems unusually lethargic, is feeding poorly, or has fewer wet diapers than expected (fewer than 6 wet diapers a day after day 4 is a common guideline, though your care team may give you specific targets).
- You are concerned for any reason, including persistent fussiness, vomiting after most feeds, or visible signs of discomfort during feeding.
The American Academy of Pediatrics recommends well-child visits at 2 to 4 days after hospital discharge (specifically to check weight), then at 1 month, 2 months, 4 months, 6 months, 9 months, and 12 months in the first year. Each visit includes a weigh-in plotted on the growth chart. Following this schedule is the most reliable way to catch growth concerns early.