Baby Head Circumference: What to Know

Head circumference is one of the three key growth measurements tracked at every well-baby visit. It provides important information about brain growth during the critical first years of life. Here is what the numbers mean and when changes warrant attention.

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Key takeaways

  • Head circumference reflects brain growth and is measured until age 2 to 3
  • The brain reaches approximately 80% of adult size by age 2 and 90% by age 5
  • Head grows about 12 cm in the first year (2 cm/month for 3 months, then 1 cm/month for 3 months, then 0.5 cm/month)
  • Familial macrocephaly (large head running in families) is the most common cause of a large head
  • Rapid crossing of percentile lines, either up or down, warrants evaluation

Normal head growth by age

Birth to 3 months

The head grows approximately 2 cm per month during this period of rapid brain development. Average head circumference at birth is about 34 to 35 cm (13.4 to 13.8 inches) for full-term babies.

3 to 6 months

Growth rate slows to approximately 1 cm per month.

6 to 12 months

Growth slows further to about 0.5 cm per month. By 12 months, head circumference has increased approximately 12 cm from birth, reaching about 46 to 47 cm (18 to 18.5 inches).

12 to 24 months

The head grows about 2 cm during the entire second year, reaching approximately 48 to 49 cm by age 2. After age 2, head growth slows considerably and is not routinely measured at every visit.

How head circumference is measured

Head circumference (also called occipitofrontal circumference or OFC) is measured by placing a flexible, non-stretchable tape around the largest part of the head. The tape goes across the forehead just above the eyebrows and ears, and around the most prominent part of the back of the head (the occiput). The measurement should be taken at least twice and the largest value recorded. Hair accessories, braids, and thick hair can affect accuracy.

Macrocephaly: when the head is large

Macrocephaly is defined as head circumference above the 97th percentile (or greater than 2 standard deviations above the mean). The most common cause is benign familial macrocephaly, where one or both parents have a large head. This is diagnosed when the head is large but growing at a normal rate, the fontanelle is not bulging, and development is normal. Parental head circumference should be measured.

Causes that require evaluation include hydrocephalus (excess cerebrospinal fluid), certain genetic syndromes, and megalencephaly (enlarged brain). Signs that warrant urgent evaluation include a tense or bulging fontanelle, rapid increase crossing percentile lines, vomiting (especially in the morning), sun-setting eyes, or developmental regression.

Microcephaly: when the head is small

Microcephaly is defined as head circumference below the 3rd percentile (or more than 2 standard deviations below the mean). It can be present at birth (congenital) or develop postnatally (acquired). Congenital causes include genetic conditions, prenatal infections (such as Zika, CMV, or rubella), fetal alcohol exposure, and chromosomal abnormalities. Acquired microcephaly may result from birth injury, infection, or conditions affecting brain growth.

Not all babies with head circumference below the 3rd percentile have microcephaly in the clinical sense. Some are constitutionally small, particularly if parents have smaller heads. Evaluation typically includes neurological assessment, developmental screening, and sometimes imaging.

Average head circumference by age (WHO 50th percentile)

AgeBoys (median)Girls (median)
Birth34.5 cm (13.6 in)33.9 cm (13.3 in)
3 months40.5 cm (15.9 in)39.5 cm (15.6 in)
6 months43.3 cm (17.0 in)42.2 cm (16.6 in)
9 months45.0 cm (17.7 in)43.8 cm (17.2 in)
12 months46.1 cm (18.1 in)44.9 cm (17.7 in)
24 months48.3 cm (19.0 in)47.2 cm (18.6 in)

Frequently asked questions

Why do pediatricians measure head circumference?
Head circumference is a proxy for brain growth. The brain grows rapidly during infancy, and the skull expands to accommodate it. Tracking head circumference helps identify potential issues like hydrocephalus (fluid buildup causing rapid head growth), craniosynostosis (premature fusion of skull sutures), or conditions that may slow brain growth. It is measured routinely at every well-child visit until age 2 or 3.
My baby has a large head. Should I be concerned?
A large head (above the 90th percentile) is most often familial, meaning one or both parents also have larger-than-average heads. This is called benign familial macrocephaly. If your baby's head has always tracked along a high percentile, is developing normally, and has no signs of increased intracranial pressure (bulging fontanelle, vomiting, irritability), it is likely their normal. However, if head circumference is rapidly crossing percentile lines upward, your pediatrician will want to evaluate further.
When does the fontanelle (soft spot) close?
The anterior fontanelle (the larger soft spot on top) typically closes between 9 and 18 months, with an average closure around 12 to 14 months. The posterior fontanelle (the smaller one at the back) usually closes by 2 to 3 months. Early closure does not necessarily indicate a problem but should be monitored alongside head circumference measurements.
Can head shape affect circumference measurements?
Yes. Positional plagiocephaly (flat spots from lying in one position) can make the head appear larger in one direction but does not significantly affect circumference. However, craniosynostosis (premature fusion of skull plates) can restrict growth in one direction and cause compensatory growth in another, potentially affecting circumference patterns. Consistent measurement technique is important.
Is head circumference related to intelligence?
Within the normal range, head circumference is not a predictor of intelligence. A baby with a head at the 25th percentile is not less intelligent than one at the 75th percentile. What matters is that head growth follows a consistent pattern, indicating normal brain development. Extreme deviations from the norm (very small or very large) may be associated with certain conditions, but percentile position within the normal range does not predict cognitive outcomes.

Bottom line

Head circumference tracks brain growth during your baby's first years. Like weight and length, what matters most is a consistent pattern over time rather than any single number. A large or small head that runs in the family and follows a steady curve is usually nothing to worry about. Rapid changes in the growth pattern are what prompt further evaluation.

Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.

All content follows our editorial policy and is reviewed against published clinical guidelines.

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