Neural tube defects (NTDs) are birth defects that result from incomplete closure of the embryonic neural tube, leading to malformations of the brain and spinal cord. The two most common types are anencephaly (absence of major portions of the brain and skull) and spina bifida (incomplete closure of the spine and spinal cord). Other forms include encephalocele (protrusion of brain through a skull defect) and iniencephaly (rare, severe defect of the occipital bone). On prenatal ultrasound, anencephaly can be diagnosed as early as 11 to 13 weeks, appearing as absence of the skull vault above the orbital ridge. Spina bifida is detected by abnormalities of the spine and associated findings like lemon sign and banana sign. Associated hydrocephalus is common. Elevated maternal serum alpha-fetoprotein can prompt targeted ultrasound screening for NTDs.
Sonography Term