From the Guidelines
Prenatally, microcephaly is defined as a head circumference that measures more than 2 standard deviations below the mean for gestational age on ultrasound examination. This definition is based on the most recent and highest quality study available, which emphasizes the importance of accurate measurement and comparison to gestational age and sex-specific norms 1. The diagnosis of microcephaly is typically made during routine prenatal ultrasound screenings, which are commonly performed at 18-22 weeks of gestation, though it may not be detectable until later in pregnancy as the growth discrepancy becomes more apparent.
Key Considerations
- Microcephaly can result from genetic abnormalities, chromosomal disorders, congenital infections (such as Zika virus, cytomegalovirus, or toxoplasmosis), maternal alcohol consumption, or exposure to certain drugs or toxins 1.
- Early detection is crucial as it allows for additional testing to determine the underlying cause and helps healthcare providers counsel parents about potential outcomes and prepare for specialized care the infant may need after birth.
- The clinical outcome cannot be predicted by head size alone and largely depends on the underlying cause and the appearance of the brain on MRI 1.
Diagnostic Approach
- Accurate perinatal history-taking aids the identification of teratogen exposure and infections.
- Brain scans should be scrutinized for signs of fetal injury, including gliosis, cysts, and calcifications.
- A detailed eye examination should be routinely performed in every individual with microcephaly to identify potential ophthalmological abnormalities 1.
From the Research
Definition of Microcephaly
- Microcephaly is defined as a fetal head circumference (HC) 3SD below the mean for gestational age according to Jeanty et al.'s reference range 2.
- Alternatively, microcephaly can be defined as an occipitofrontal circumference (OFC) of the head that is less than two standard deviations below the average for age (or gestational age, if identified prenatally) and sex 3.
- Severe microcephaly is defined as an OFC that is less than three standard deviations below the average 3.
Prenatal Diagnosis of Microcephaly
- The prenatal diagnosis of microcephaly, particularly in cases of primary microcephaly, is usually difficult before the 3rd trimester 4.
- Prediction of microcephaly at birth based on conventional prenatal biometry is associated with a high percentage of false positive diagnoses 2.
- Integrating additional parameters, such as stricter fetal HC cut-offs, small-for-gestational age (SGA), decreased HC/abdominal circumference and HC/femur length ratios, presence of associated malformations, and family history, can improve prediction 2.
- The use of a new reference range for vertical cranial dimensions, such as the foramen magnum-to-cranium distance (FCD), can help exclude fetuses with a small HC associated with a vertical cranial deformity without missing those with actual microcephaly 2.
Challenges in Prenatal Diagnosis
- The prenatal diagnosis of microcephaly is not excluded by normal biometry on second trimester sonography 5.
- The lack of knowledge regarding the prognosis when microcephaly is found during pregnancy makes counseling difficult 4.
- The degree of microcephaly severity is significantly over-estimated compared to the corresponding microcephaly at birth 2.