Fetal small head circumference is not always caused by a genetic disorder. In some cases, microcephaly runs in families (a condition known as benign familial microcephaly), in these cases there are no other symptoms, such as intellectual disability.
In addition, a small head circumference may be caused by damage to the developing fetal brain tissue during pregnancy (such as impaired blood supply to the fetal brain, intrauterine infections, and more).
However, there are certain cases when microcephaly indicates a genetic abnormality. In these cases, it will not be an isolated condition.
Should every finding of microcephaly raise concern?
A head circumference value that is considered small but still in the normal range is relatively common and usually has no medical significance, especially when one or more of the parents also has a smaller-than-average head circumference.
Moreover, a head circumference that is two standard deviations (SDs) smaller than the average for gestational age, which appears in three percent of fetuses, is not considered a cause for concern if it is an isolated finding (related to normal findings, especially when one of the parents has a head circumference that is smaller than average).
However, microcephaly can be considered a pathological finding when the following conditions occur – the head circumference was normal but began to deteriorate over time (acquired microcephaly); when the deviation is severe (usually three SDs under average); and when a large amount of fluid is detected in or around the brain.
This is especially true in cases of a family history of microcephaly with intellectual disability, and in cases of additional symptoms including congenital malformations in various organs or the brain.
What genetic disorders can cause microcephaly?
- Chromosome abnormalities, such as a deletion in the short arm of chromosome 5.
- Autosomal recessive microcephaly accompanied by intellectual disability.
- Certain genetic disorders that cause delayed development of fetal brain tissue.
- Genetic disorders that cause neural tube defects (NTD).
- Rare genetic diseases that cause a postnatal regression in brain volume, such as Reiter’s syndrome or Angelman syndrome.
Besides the aforementioned diseases, there are hundreds of inherited disorders linked to a smaller head circumference, such as congenital bone disorders, albinism, autoimmune disorders, and more.
How is microcephaly inherited?
Familial microcephaly has an autosomal dominant inheritance pattern, while microcephaly accompanied by intellectual disability has an autosomal recessive inheritance pattern. When microcephaly is accompanied by other congenital defects that are part of certain genetic diseases, the inheritance pattern varies and depends on the type of disorder.