A baby's head is called large (macrocephaly) when its circumference is more than 2 standard deviations (SD) above average for age and sex (StatPearls), and small (microcephaly) when it's more than 2 SD below average. Below 3 SD is severe microcephaly (AAN practice parameter). On the WHO charts US pediatricians use, the 2 SD cutoffs are the lines printed as the 98th and 2nd percentiles, exactly the 97.7th and 2.3rd (CDC).
Being past one of those lines isn't a diagnosis. A big or small head that follows its own curve, in a baby who is developing well and whose parents have similar-sized heads, is usually just family size. What needs a closer look is a head that crosses percentile lines, a soft spot that stays bulging, or delays in development.
What counts as a big or small head?
| Term | Head circumference | Roughly equal to |
|---|---|---|
| Macrocephaly | More than 2 SD above average | Above the 97.7th percentile (printed as 98th) |
| Macrocephaly some specialists call clinically relevant | More than 3 SD above average | Above about the 99.9th percentile |
| Microcephaly | More than 2 SD below average | Below the 2.3rd percentile (printed as 2nd) |
| Severe microcephaly | More than 3 SD below average | Below about the 0.1st percentile |
Source: StatPearls; Accogli et al. 2022; AAN practice parameter; CDC
For a 6-month-old boy, the 2 SD range runs from 40.9 cm (16.1 in) to 45.8 cm (18.0 in). Our baby head circumference chart has the full WHO tables by month for boys and girls.
Large heads are common. Macrocephaly affects up to 5% of children, and about 2–3% of healthy people have a head between 2 and 3 SD above average. That's why some specialists only call macrocephaly "clinically relevant" above 3 SD (Accogli et al. 2022). Microcephaly is rarer: the CDC estimates about 1 in 1,150 US babies has it (CDC).
Head size is measured at every well-child visit through 24 months (Bright Futures) because it's a stand-in for brain growth (AAP).
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Why most large heads are harmless
Benign familial macrocephaly. Big heads run in families. A review of how to evaluate large heads found that isolated benign head enlargement is a family trait in over 50% of cases, and up to 60% of children with mild macrocephaly (2 to 3 SD) have a family history of it. Its advice: always measure the parents' heads (Accogli et al. 2022). The typical pattern is a head on a high but steady curve, in a baby whose development and exam are normal.
Benign enlargement of the subarachnoid spaces (BESS). Also called benign external hydrocephalus, this is extra fluid in the spaces around the brain. It affects about 0.5 per 1,000 babies, more often boys. The head grows quickly around 6 months, levels off at about 18 months, and the condition resolves on its own by about age 3. Most babies have no signs of raised pressure, and a head ultrasound through the open soft spot can be the first test. Some studies report motor or speech delays, so development is followed (Accogli et al. 2022).
Less common causes include hydrocephalus (fluid building up inside the skull), bleeding inside the skull, and rare genetic and metabolic conditions (MedlinePlus).
A steady high curve vs a head that's crossing lines
The trend tells you far more than one measurement. Here's a boy whose head is large but steady:
Head circumference-for-age, boys, birth to 2 years
His head measures 37.2 cm at birth, at the 98th percentile, and stays there: the 98th at 6 months, the 98th at 12 months and the 98th at 2 years. Technically that's macrocephaly the whole time, but the curve runs alongside the printed lines. If his development is on track and a parent also has a large head, this is the classic pattern of benign familial macrocephaly.
Now a boy who starts at average and speeds up:
Head circumference-for-age, boys, birth to 12 months
He's at the 51st percentile at birth, the 72nd at 2 months, the 88th at 3 months, the 97th at 4 months, and above the 99th percentile by 5 months. His head grew 3.5 cm between 3 and 5 months, when about 1 cm a month is typical at that age (StatPearls). A head that rapidly crosses two major percentile lines is one of the triggers for brain imaging in published approaches (Accogli et al. 2022). The cause may turn out to be BESS, which is usually benign, or hydrocephalus, which needs specialist care. Only an exam, and often an ultrasound, can tell them apart.
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Baby Head Growth Velocity Calculator
Enter two head measurements with dates to see how fast your baby's head is growing compared with WHO norms.
When a baby's head is small
The cause of microcephaly is unknown in most babies. Known causes include some infections during pregnancy, poor nutrition, alcohol or certain drugs during pregnancy, and problems with blood flow to the developing brain (CDC), as well as genetic conditions such as Down syndrome and Zika infection during pregnancy (MedlinePlus). Depending on severity, babies may have seizures, developmental delay, feeding problems, hearing loss or vision problems.
Small heads can run in families too. A pediatric review notes that measuring the parents' heads helps diagnose familial microcephaly, and that isolated or familial microcephaly hasn't been linked to lower developmental outcomes or IQ when there are no other problems. If a small head is in proportion to length and weight, and the baby has no neurological signs, the usual plan is watching development closely rather than testing (Hanzlik and Gigante 2017).
Severity matters. Children with severe microcephaly are more likely to have abnormal brain scans (about 80%) than those with milder microcephaly (about 40%) (AAN practice parameter). So does a head whose growth slows: doctors ask whether the head is growing more slowly than the body (MedlinePlus). Slow or no head growth is also a sign of craniosynostosis, where skull seams fuse early (CDC). Our flat head syndrome guide explains how that differs from a harmless flat spot.
What your pediatrician checks
- Repeats the measurement. Heads are hard to measure in a wriggly baby with thick hair, and any abnormal reading should be confirmed (StatPearls).
- Looks at the trend across visits, and at length and weight.
- Measures the parents' heads and asks about family history.
- Examines your baby: the soft spot and skull seams, head shape, development and a neurological exam. The soft spot guide covers what's normal there.
- Orders imaging if needed. For a large head, that may be a head ultrasound while the soft spot is open, or a CT or MRI (MedlinePlus; Accogli et al. 2022). For a small head, MRI is often the first test, then genetic testing (Hanzlik and Gigante 2017).
If you're worried about development as well as head size, our guide to signs of developmental delay lists what to bring up.
How to measure your baby's head at home
This is the technique the AAP describes for clinics (AAP):
- Use a narrow tape that doesn't stretch.
- Take out hair clips, bands or anything else in the way.
- Place the tape on the forehead just above the eyebrows and wrap it around the widest part of the back of the head, level on both sides.
- Pull it snug enough to press the hair against the skull.
- Slide it slightly up and down to find the largest measurement, and write it down to the nearest millimeter (0.1 cm).
Home numbers vary more than clinic ones, so compare the pattern over several months rather than reacting to a single reading. Babies born early are plotted by corrected age through age 2 (see the premature baby growth chart guide). GrowthKit plots each head measurement on the WHO chart up to age 5 and shows its percentile, so a curve that's drifting across lines is easy to spot and to share with your pediatrician as a PDF report.
Free tool
Baby Head Circumference Percentile Calculator
Enter your baby's age, sex and head circumference to see their exact WHO percentile.
When to call your pediatrician about head size
| What you notice | What to do |
|---|---|
| Head is large or small but follows the same percentile line, and development is on track | Usually normal; keep measuring at well-child visits |
| Head is above the 98th or below the 2nd percentile line but steady | Mention it at the next visit (sooner if it's new); bring both parents' head measurements |
| Head has crossed two major percentile lines, up or down, or hasn't grown between visits | Call your pediatrician and ask when it should be rechecked |
| Head growing more slowly than length and weight, a hard ridge along a seam, or an unusual head shape | Call your pediatrician |
| A large or small head plus delayed development, lost skills or feeding problems | Call your pediatrician |
| Soft spot stays bulging while your baby is calm and upright, especially with fever or unusual drowsiness | Seek emergency care now |
| Fast-growing head with vomiting, extreme sleepiness or irritability, a high-pitched cry, eyes that stay turned downward, or a seizure | Call 911 or go to the emergency room |
Source: MedlinePlus: bulging fontanelles; MedlinePlus: hydrocephalus; CDC: growth chart cutoffs; Accogli et al. 2022; CDC: craniosynostosis; CDC: microcephaly
Most babies with a head near the top or bottom of the chart turn out to be healthy children with their family's head size. The measurement that deserves attention is the one that breaks away from its curve.
