One of the major measurements that gynecologists take during an ultrasound is the circumference of your baby’s head. This has become as common as measuring weight and body length. The size of your baby’s head in conjunction with its gestational age can actually tell your doctor if there are any major developmental problems.

Your gynecologist will take this measurement in all of your ultrasounds once the fetus is big enough for the head to be measured. The size of the baby’s head can tell the doctor if it potentially has medical conditions related to malformation, such as microcephaly, macrocephaly and hydrocephalus.

Reference chart for head circumference at each gestational week

As your baby develops there are different averages expected for head circumference. Your doctor will take into account the averages, as well as the maximum and minimum acceptable sizes depending on your gestational week. They will also look at the overall size of your baby, and how it has been developing over the weeks.

It’s important to remember that these sizes are a guide line and don’t necessarily mean that there is anything wrong with your baby. Your gynecologist will run further tests if they suspect that there are any problems.

Week of gestation Minimum Size Average Size Maximum Size
12 weeks 4,2cm 6,8cm 8,8cm
13 weeks 5,6cm 8,2cm 10,2cm
14 weeks 7,1cm 9,7cm 11,7cm
15 weeks 8,4cm 11,0cm 13,0cm
16 weeks 9,8cm 12,4cm 14,4cm
17 weeks 11,2cm 13,8cm 15,8cm
18 weeks 12,5cm 15,1cm 17,1cm
19 weeks 13,8cm 16,4cm 17,1cm
20 weeks 15,7cm 17,7cm 19,7cm
21 weeks 16,3cm 18,9cm 20,9cm
22 weeks 17,5cm 20,1cm 22,1cm
23 weeks 18,7cm 21,3cm 23,3cm
24 weeks 19,8cm 22,4cm 24,4cm
25 weeks 20,9cm 23,3cm 25,5cm
26 weeks 22,0cm 24,6cm 26,6cm
27 weeks 23,0cm 25,6cm 27,6cm
28 weeks 24,0cm 26,6cm 28,6cm
29 weeks 24,9cm 27,5cm 29,5cm
30 weeks 25,8cm 28,4cm 30,4cm
31 weeks 26,4cm 29,3cm 31,3cm
32 weeks 27,5cm 30,1cm 32,1cm
33 weeks 28,2cm 30,8cm 32,8cm
34 weeks 28,9cm 31,5cm 33,5cm
35 weeks 29,6cm 32,2cm 34,2cm
36 weeks 30,2cm 32,8cm 34,8cm
37 weeks 30,7cm 33,3cm 35,5cm
38 weeks 31,2cm 33,8cm 35,8cm
39 weeks 31,6cm 34,2cm 36,2cm
40 weeks 32,0cm 34,6cm 36,6cm

After birth, a newborn baby should have a head circumference of between 33cm and 38.6cm. If it is below this range, your doctor will look for microcephaly (reduction of the brain). Above this range, your doctor will look for macrocephaly (enlargement of the brain) and hydrocephalus (water in the cranial cavity).

The main causes of being outside of the acceptable average range

There are many reasons why your baby’s head circumference could be outside of what is considered the acceptable average range. One of the most common is the mother contracting certain diseases while pregnant. These include toxoplasmosis, rubella and the zika virus. There are also several genetic diseases that can cause a baby’s head circumference to be outside of the normal range. These include Edwards’ syndrome, Down syndrome and others.

Unfortunately, there is no cure or treatment for an increased head circumference. This is why it is important for your doctor to run all of the necessary tests to confirm whether or not an enlarged head is a symptom of a disease. The most common extra test is an amniocentesis. It is an invasive test, and your doctor will only recommend it if they feel it is necessary.

What a smaller or larger head circumference means for you and your child

A baby with either a smaller or larger head circumference than average can mean that they will have certain limitations put on their lives. In the more extreme cases, your child will likely need constant monitoring and special medical assistance throughout their lives because of restricted mental and physical development.

It’s important to remember that there are degrees to which your child can be affected. If they are only slightly outside of the normal range, they may be able to live reasonably normal lives. Talk to your doctor about what you can expect once a diagnosis has been reached, so that you can start to prepare for your child’s needs once it is born.

Readers’ questions:

If my gynecologist discovered that my baby’s head isn’t the normal size, what should I do?

The first step is to listen to your gynecologist and go for the tests that they recommend. They will be able to give you a full diagnosis with these tests and help you to prepare for what to expect once your baby is born. It’s important to try not to panic or despair until a full diagnosis has been found and then to talk to your doctor about what you should do.