Decreased head circumference

What it means

An abnormally reduced head circumference in a growing child. Head circumference is measured with a nonelastic tape and comprises the distance from above the eyebrows and ears and around the back of the head. The measured HC is then plotted on an appropriate growth chart. Microcephaly is defined as a head circumference (HC) that is great than two standard deviations below the mean of age- and gender-matched population based samples. Severe microcephaly is defined with an HC that is three standard deviations below the mean.

Although HC measures skull size, it typically also reflects overall brain volume and has been described as a widely used proxy of neural growth and brain size. Brain size outside of normal values is an important risk factor for cognitive and motor delay. Microcephaly at birth has been termed primary microcephaly and that acquired after birth is secondary microcephaly.

Rare diseases that can present with this6

The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions. A sign on this page is a conversation to have with a clinician, not a conclusion.

Part of the broader category