= lethal anomaly with failure of closure of the rostral end of the neural tube by 5.6 weeks MA

◊Associated with highest AF-AFP and MS-AFP values; >90% will be detected with MS-AFP ≥2.5 MoM

Incidence: 1÷1,000 births in USA (3.5÷1,000 in South Wales); M÷F = 1÷4; most common congenital defect of CNS; 50% of all neural tube defects

Recurrence rate: 3–4%

Etiology: multifactorial (genetic + environmental)

Path: absence of cerebral hemispheres + cranial vault; partial / complete absence of diencephalic + mesencephalic structures; hypophysis + rhombencephalic structures usually preserved

Risk factors: family history of neural tube defect; twin pregnancy

Associated anomalies:

• spinal dysraphism (17–50%), cleft lip / palate (2%), clubfoot (2%), umbilical hernia, amniotic band syndrome

• absence of bony calvarium cephalad to orbits
• ± cranial soft-tissue mass (= angiomatous stroma)
• bulging froglike eyes
• short neck
• polyhydramnios (40–50%) after 26 weeks GA (← failure of normal fetal swallowing) / oligohydramnios

Dx: in 100% >14 weeks GA

Prognosis: uniformly fatal within hours to days of life; in 53% premature birth; in 68% stillbirth

DDx: acrania, encephalocele, amniotic band syndrome