A topic that has come up recently is the importance of measuring the baby’s head during prenatal care, in addition to the commonly measured weight and size. This is because, through a simple ultrasound, specialized doctors are able to detect health and developmental issues by measuring head circumference while the baby is still in utero. Thus, in a routine obstetric ultrasound it is possible to check and analyze the size of the baby’s developing skull.

The measurement of head circumference at a certain stage of pregnancy helps identify changes in head size early on. Problems such as microcephaly, macrocephaly, hydrocephaly and some other malformations, for example, can be easily detected1.

The ideal head circumference for babies depends on the gestational age. Development occurs slowly and the measurements change as the weeks of pregnancy progress. However, after birth a head circumference considered “normal” ranges from above 33 up to 38.6 cm. Below this measurement, doctors consider it to be microcephaly (reduced brain size). Above this measurement, it is considered macrocephaly (enlarged brain) or even hydrocephaly (fluid in the skull).

Main Causes of Changes

The causes of changes in head circumference can be varied, including diseases the mother contracts during pregnancy, such as toxoplasmosis, rubella, and Zika virus. Changes in head circumference can also be caused by genetic conditions such as Edwards syndrome, Down syndrome and others2.

Increase in head circumference, if confirmed, cannot be reversed, as there is no cure or treatment. However, it is very important to have proper follow-up throughout pregnancy and talk to your doctor about other possible tests that may be done to determine the reason for the change and what type of issue the baby may have. Some additional tests, such as amniocentesis, may help with the diagnosis, but because they are invasive procedures, they are prescribed with caution.

There are degrees of abnormality in skull development; some babies may have milder changes, such as a measurement that would normally be 33 cm but is reduced to 31 cm, for example. However, children with more pronounced head circumference changes—greater or smaller—may end up with limitations for the rest of their lives, such as microcephaly, which leads to intellectual, physical, and motor limitations. In addition, these children will require special and ongoing care from various professionals such as speech therapists, physical therapists, neurologists, and others.

Reference Table for Head Circumference

We should remember that the table is just a reference. For a conclusive diagnosis, the measurements must be evaluated by a doctor. If you have any questions, please see your obstetrician3.

Gestational Weeks Minimum Size Average Size Maximum Size
12 weeks 4.2 6.8 8.8
13 weeks 5.6 8.2 10.2
14 weeks 7.1 9.7 11.7
15 weeks 8.4 11.0 13.0
16 weeks 9.8 12.4 14.4
17 weeks 11.2 13.8 15.8
18 weeks 12.5 15.1 17.1
19 weeks 13.8 16.4 17.1
20 weeks 15.7 17.7 19.7
21 weeks 16.3 18.9 20.9
22 weeks 17.5 20.1 22.1
23 weeks 18.7 21.3 23.3
24 weeks 19.8 22.4 24.4
25 weeks 20.9 23.3 25.5
26 weeks 22.0 24.6 26.6
27 weeks 23.0 25.6 27.6
28 weeks 24.0 26.6 28.6
29 weeks 24.9 27.5 29.5
30 weeks 25.8 28.4 30.4
31 weeks 26.4 29.3 31.3
32 weeks 27.5 30.1 32.1
33 weeks 28.2 30.8 32.8
34 weeks 28.9 31.5 33.5
35 weeks 29.6 32.2 34.2
36 weeks 30.2 32.8 34.8
37 weeks 30.7 33.3 35.5
38 weeks 31.2 33.8 35.8
39 weeks 31.6 34.2 36.2
40 weeks 32.0 34.6 36.6

Readers’ Questions

What should I do if there is an alteration in head circumference?

If there is any change, the obstetrician will inform you and request tests for confirmation. If necessary, complementary exams will be conducted for a more accurate assessment of the case. The best course of action is not to panic, wait for confirmation from the subsequent reports and the specialist’s opinion. No diagnosis of this level is made with just one test, so if in doubt, seek a second medical opinion.

Also see: Reference Table of Fetal Size and Weight During Pregnancy