Head circumference: the measurement families ignore and doctors do not

Why it is measured

Head circumference tracks brain growth, which is fastest in the first two years — the brain roughly doubles in size in the first year. A head that is growing along its curve is the simplest available evidence that the brain is growing normally.

It is measured at every routine visit until about two years, around the widest part of the head, and plotted on the same chart as weight and length.

As with the other measurements, the curve matters far more than a single number. A baby steadily on the 20th percentile for head size is normal; a baby crossing rapidly upward or downward through percentile lines is what prompts a closer look.

The soft spots

The posterior fontanelle, at the back, closes by about two to three months. The anterior fontanelle, the diamond on top, usually closes between nine and eighteen months — later than most families expect.

A soft spot that gently pulses is normal — that is the blood flow. Routine washing, oiling and combing are all safe; the membrane covering it is tough.

A bulging, tense fontanelle in a distressed baby, or a deeply sunken one in an unwell baby, are both same-day matters. The first can indicate raised pressure inside the skull, the second dehydration.

Very early closure or very late closure of the fontanelle is worth mentioning to the doctor, who will look at it alongside head growth and development rather than in isolation.

Head shape, and what not to do

Flattening on one side (positional plagiocephaly) is common in babies who always sleep with the head turned the same way. It is a shape issue rather than a brain issue, and it improves with position changes and plenty of supervised tummy time when awake.

Do not use head-shaping pillows or rings sold for babies — they add a suffocation risk and do not reliably change shape. Back-sleeping remains the rule regardless.

Do not press, mould or massage the skull to shape it, which is a common family practice and achieves nothing beyond distress.

A newborn's head is often moulded or cone-shaped after a vaginal birth, and settles within days. Swellings from birth also settle, though some take weeks.

When to see a doctor

Same-day: a bulging tense fontanelle, a rapidly enlarging head, a very sunken fontanelle in an unwell baby, a fit, persistent vomiting with irritability or sleepiness, or a baby whose eyes seem to look permanently downward.

Book an assessment for: head circumference crossing percentile lines up or down, a head much larger or smaller than the curve suggests, persistent flattening that is not improving with position changes, a ridge along a suture line, or any developmental concern alongside head-size changes.

This article is general information, not medical advice, and is no substitute for personal medical advice. For any decision about your or your child's health, please consult your doctor.

Frequently asked questions

Q: My baby's head looks large. Should I worry? — Head size is partly familial, and a large head that grows along its own curve with normal development is usually just a family trait. It is the crossing of lines, not the position, that matters.

Q: The soft spot has not closed at fourteen months. — Within the normal range, which runs to about eighteen months. Mention it at the next visit for reassurance.

Q: Can tummy time really fix a flat head? — It genuinely helps, along with varying which end of the cot the baby's head is placed and reducing time in car seats and bouncers. Severe or unimproving cases are referred for assessment, and helmets are used in a small minority.

Q: Is it safe to comb over the soft spot? — Yes. Normal handling, washing, oiling and combing are all fine. Pressure and pushing are not.