Head circumference in babies: why it is measured
At health checks and developmental reviews, your health visitor or GP will measure your baby in several ways. One of those measurements is head circumference, and if you have ever wondered why it is taken and what it shows, this article walks through the basics using NHS guidance.
Why head circumference is measured
Head circumference is one of the standard measurements taken during your baby's health and development reviews. Together with weight and length, it gives healthcare professionals a picture of how your baby is growing overall. Steady, consistent growth across all three measurements is one of the signs that a baby is healthy and developing well.
Because the brain grows rapidly in the first months of life, tracking head size over time can give clinicians useful information. A single measurement on its own means very little; it is the pattern of growth over several checks that matters most.
When is head circumference measured?
According to NHS guidance, head circumference is measured as part of the 6-8 week review. At this review a GP carries out a thorough physical examination. As well as head circumference, the review includes:
- Weight and length recorded and plotted in the red book
- Eyes checked using a special torch
- Heart listened to for any irregularities
- Hips checked to confirm they have developed properly
- Testicles checked in boys to confirm they have descended into the scrotum
Head circumference may also be measured at other developmental reviews during the first year, depending on your local health visiting team and any concerns that arise. Your health visitor will let you know what is planned at each visit.
The red book and centile charts
All measurements taken at health and development reviews are recorded in your baby's personal child health record, which most parents know as the red book. This small booklet stays with you and travels to every appointment, so every professional your baby sees has a complete record.
Inside the red book you will find centile charts. NHS guidance describes these charts as showing the average weight and height gain for babies of different ages. The curved lines printed across each chart are called centile lines, and each line represents a particular point in the range of typical measurements for babies that age.
When a measurement is plotted on the chart, it shows roughly how your baby's growth compares with other babies of the same age. Most babies follow a path that stays close to one of the centile lines over time. Importantly, separate charts are used for boys and girls, because NHS guidance notes that boys tend to be a little heavier and taller than girls. The same principle applies to head circumference centile charts.
What the centile lines mean in practice
It is very common for a baby's plotted measurements to move slightly up or down the chart between checks. NHS guidance explains that measurements may go up or down by one centile line and that this is generally not a cause for concern.
What is less common, and worth discussing with your health visitor, is a measurement that crosses two or more centile lines in either direction. If that happens, the NHS advises talking to your health visitor, who can assess whether any follow-up is needed. They may simply want to monitor the measurement more closely, or they may arrange further checks.
A single measurement sitting at the higher or lower end of the chart does not in itself indicate a problem. What matters is the overall pattern of growth and how measurements are changing over time.
Growth and development reviews in the first year
Reviews are offered at regular intervals throughout the first two years and are usually carried out by health visitors or members of their team. They can take place at your home, your GP surgery, a baby clinic or a children's centre. At each review, your health visitor will ask how you and your baby are getting on, discuss feeding and development, and record measurements in the red book.
For weight specifically, NHS guidance sets out how often babies should be weighed when there are no concerns:
- Up to 6 months: no more than once a month
- 6 to 12 months: no more than every two months
- Over 12 months: no more than every three months
Babies may be weighed more often if a health visitor or parent has a specific concern. The same review framework covers all measurements including head circumference, and your health visiting team will advise if more frequent checks are appropriate for your baby.
What to do if you have a concern
If you notice that your baby's head looks or feels different, is growing very quickly, or if you have any worry about your baby's development, the right step is to contact your health visitor or GP and describe what you have noticed. They are best placed to assess your individual baby, look at the growth chart in the red book, and advise on next steps.
It is always worth raising concerns rather than waiting. Health visitors are used to parents asking questions at any point between scheduled reviews, not only at formal appointments.
Frequently asked questions
When is a baby's head circumference measured?
Head circumference is measured as part of the 6-8 week review, which also includes weight, length and physical checks of the eyes, heart, hips and (for boys) testicles. It may also be recorded at other developmental reviews during the first year.
Where are head circumference measurements recorded?
Measurements are recorded in the personal child health record, which is commonly known as the red book. The red book contains centile charts that let you and your health visitor see how your baby's head size compares with other babies of the same age over time.
What are centile lines on a growth chart?
Centile lines are the curved lines printed on the growth charts inside the red book. NHS guidance describes them as showing the average weight and height gain for babies of different ages. The same principle applies to head circumference charts: each line represents a point in the range of typical measurements for that age.
Should I be worried if my baby's measurements cross a centile line?
NHS guidance says measurements may go up or down by one centile line, and that is generally expected. It is less common for measurements to cross two centile lines. If that happens, the NHS advises talking to your health visitor, who can look at the full picture and advise whether any follow-up is needed.
Do boys and girls have the same head circumference charts?
No. Separate growth charts are used for boys and girls. NHS guidance notes that boys tend to be a little heavier and taller than girls, and this difference is reflected in the centile charts used for all measurements, including head circumference.
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