- 1 in 1,000 births.
- 80% of affected infants have a normal head circumference at birth and 90% of affected individuals had a normal head circumference in the second trimester.
- Ultrasound diagnosis is made usually in the late second and third trimesters.
- The fetal head circumference to abdominal circumference ratio is below the 3rd percentile (2 standards deviations below the normal mean for gestational age).
- There is slopping forehead due to the disproportion of the frontal lobes and the face.
- In most cases presenting in the second trimester there is associated holoprosencephaly or encephalocele and in those presenting in the third trimester there are abnormalities of sulcation or neuronal migration.
- Chromosomal abnormalities are rare and the most common are trisomies 13, 18 and 21.
- Genetic syndromes are very common, most of them being caused by single gene defects with either autosomal recessive or X linked patterns of inheritance. The most common are: Meckel-Gruber, Walker-Walburg, Miller-Diecker, Smith-Lemli-Opitz, Seckel syndrome.
- Detailed ultrasound examination, including neurosonography.
- Invasive testing for karyotyping and array.
- TORCH test for fetal infections.
- Fetal brain MRI at ≥32 weeks’ gestation for diagnosis of abnormalities of neuronal migration, such as lissencephaly and polymicrogyria.
- Ultrasound scans every 4 weeks to monitor the evolution of head circumference.
- Standard obstetric care and delivery.
- Isolated: the risk of neurodevelopmental delay inreases with decreasing head circumference from 10% if the circumference is 2 to 3 standard deviations (SD) below the normal mean for gestational age, to 100% if >4 SD's.
- Syndromic: the prognosis depends on the underlying condition.
- No associated structural defects: 5-10%.
- Familial form of isolated microcephaly: 25%.