A term newborn averages ~3.3 kg (7 lb 4 oz), ~50 cm long, and ~35 cm head size, with wide healthy ranges by sex and gestation.
Average Newborn Size Guide: Weight, Length, Head
Newborns don’t arrive in a single mold. In the delivery room, staff record three core numbers: weight, crown-to-heel length, and head circumference. These give a quick picture of body size at birth.
The figures below reflect term, singleton babies. They match what hospitals and pediatric groups use day to day. Global reference curves from the INTERGROWTH-21st project also line up closely with these yardsticks, and are widely used beside local charts.
| Measure | Average (Term) | Typical Range (Term) |
|---|---|---|
| Birth weight | ~3.3 kg (7 lb 4 oz) | ~2.5–4.0 kg (5 lb 8 oz–8 lb 13 oz) |
| Length | ~50 cm (19.7 in) | ~47–53 cm (18.5–20.9 in) |
| Head circumference | ~35 cm (13.8 in) | ~32–37 cm (12.6–14.6 in) |
Hospitals often quote the same ballpark numbers you see here. For a plain-language explainer that mirrors this, see the pediatric overview from Stanford Children’s Health, which lists about 7 lb for weight, 50 cm for length, and 35 cm for head size. Many clinics also use the newborn size standards published by the INTERGROWTH-21st collaboration.
What Shapes Average Newborn Size?
Gestational Age
Babies born at 37–42 weeks tend to cluster around the averages. Earlier arrivals are lighter and shorter. Care teams interpret any single number in light of weeks of pregnancy.
Sex And Family Pattern
Boys usually weigh a touch more and measure a touch longer than girls at birth. Family build matters too. Taller parents often welcome longer babies. Siblings also tend to land near one another on the chart.
Singletons Vs Multiples
Twins and higher-order multiples are commonly smaller at birth. Shared space and earlier delivery play a role.
Pregnancy Health And Habits
Diabetes in pregnancy, high blood pressure, smoking, altitude, nutrition, and some medicines can nudge birth size up or down. Providers track these factors during prenatal care and interpret newborn size with that context.
How Clinicians Measure Weight, Length, And Head Size
Weight
Right after delivery, staff weigh the baby on a calibrated scale, no clothes or diaper. A repeat weight follows within 24 hours. Small shifts in the first days are expected while fluids balance out.
Length
Length is measured with a flat board and two fixed points: head at one end, heels at the other. The baby lies flat, legs gently straightened. This method keeps measurements consistent across nurses and clinics.
Head Circumference
A flexible tape wraps above the eyebrows and around the most prominent part of the back of the head. Molding from labor can change shape briefly, so teams may repeat the reading before discharge.
Percentiles: Where Does “Average” Sit?
Percentiles rank a baby’s size among peers of the same sex and week of gestation. The 50th percentile marks the middle. Readings below the 10th are often labeled small for gestational age (SGA); above the 90th, large for gestational age (LGA). A single point is less telling than the pattern across time.
Clinics choose charts that match their population. Many use the INTERGROWTH-21st newborn size standards for term babies and Fenton charts for preterm infants. These tools supply centiles for weight, length, and head size at each week, letting staff judge whether a number is expected for that week.
Average Newborn Size By Term Week (Approximate Medians)
The table below gives practical reference points within the term window. Figures represent median birth size at each completed week. Small shifts by sex are normal.
| Gestational Week | Weight (Median) | Length (Median) |
|---|---|---|
| 37 weeks | ~2.9 kg (6 lb 6 oz) | ~49.0 cm (19.3 in) |
| 38 weeks | ~3.1 kg (6 lb 13 oz) | ~49.5 cm (19.5 in) |
| 39 weeks | ~3.3 kg (7 lb 4 oz) | ~50.0 cm (19.7 in) |
| 40 weeks | ~3.4 kg (7 lb 8 oz) | ~50.5 cm (19.9 in) |
| 41 weeks | ~3.5 kg (7 lb 12 oz) | ~51.0 cm (20.1 in) |
| 42 weeks | ~3.6 kg (7 lb 15 oz) | ~51.2 cm (20.2 in) |
Why The First Week Can Look Different
Most babies lose a little weight after birth as extra fluid shifts. Feedings settle in, and weight usually returns to the birth number by about two weeks. Length and head size don’t swing much day to day.
Reading Numbers With Context
Small But Healthy
A baby can sit near the 10th percentile and still be thriving. If exams are normal and feeding is steady, teams often keep watching and let growth tell the story.
Big Doesn’t Always Mean Macrosomia
Plenty of term babies weigh over 4 kg and do well. Staff watch blood sugar, breathing, and feeding in the first hours.
Preterm Adjustments
For babies born early, teams plot measurements on preterm charts and “correct” for prematurity during the first months. That way, growth is judged against peers with the same number of weeks since conception in care later.
Practical Ways To Picture Newborn Size
Want simple anchors? A 3.3 kg baby fits size “Newborn” diapers and many “0–3 months” onesies. A 50 cm baby often needs clothing labeled 50/56 in EU sizing.
When Measurements Need A Closer Look
Reach out to your pediatrician if feeds are hard to keep down, diapers are scarce, or weight trends down across checks. Sudden jumps or drops in head size also deserve a prompt re-measurement and a chat with the care team.
Simple Terms You’ll Hear: LBW, SGA, LGA
Birthweight under 2.5 kg is labeled low birthweight. That threshold is used in public health stats and helps teams plan early follow-up. It says nothing about a child’s future potential; it’s a flag for closer care in the newborn phase.
Small for gestational age means below the 10th percentile for that week and sex. Large for gestational age means above the 90th percentile. Some clinics also use fixed cutoffs like 4,000 g or 4,500 g to flag babies with very high birthweight. The exact cutoff varies by hospital policy.
Head-To-Body Balance
At birth, the head makes up a big share of length. A 35 cm head on a 50 cm body is typical. The chest is usually a bit smaller than the head on day one. As weeks pass, the body catches up and the proportions change fast.
Why Averages Can Vary By Place
Altitude, maternal height, nutrition, and access to prenatal care can all shift the center of the curve. Some regions report slightly lower mean weights; others slightly higher. That’s why teams rely on standards that tie size to gestational week and sex, rather than a single global number.
Measuring Between Visits
Home checks can be handy. If you weigh at home, remove clothes and a wet diaper. Use the same scale each time. For head size, place the tape just above the eyebrows and around the back bump of the skull. Write numbers down and bring them to the next visit.
Does Size Predict Delivery Or Health?
Size alone doesn’t set the birth plan. Pelvis shape, baby’s position, and health history matter more. Plenty of smaller babies arrive by cesarean. Plenty of larger babies arrive vaginally. Your team pulls many threads together, not just the weight estimate.
From Birth Size To Growth Curve
Once you leave the hospital, growth becomes the star of the show. Feeding, sleep, diapers, and steady gains matter day to day. The first weeks bring big changes, and curves smooth out as months pass. A baby who starts small can climb lines. A baby who starts big can settle into a new lane. The job is steady care and regular checkups.
Newborn Size At A Glance
Most term babies land near 3.3 kg, 50 cm, and a 35 cm head. Percentiles and gestational age help place those numbers in context. One visit rarely tells the whole story; the trend across visits does.