- Ultrasound is highly accurate for detection of anencephaly during the second trimester, with a detection rate of 100%.
- Lack of bony calvarium above the orbits.
- Absence of the cerebral hemispheres but normal midbrain and posterior fossa.
- An angiomatous mass with multiple cystic spaces replacing the forebrain; its presence must not dissuade the examiner from the correct diagnosis of anencephaly.
- In early diagnosis, spectacle sign, prominent orbits on the coronal view and may look like frog-eye face or Mickey-mouse face; the brain will have an irregular floppy outline due to the absence of the skull or a mobile cranial cyst.
- Polyhydramnios secondary to severe brain dysfunction resulting in ineffective fetal swallowing, found in half of cases and more often in late pregnancy.
- Echogenic amniotic fluid is found in nearly 90% of cases of acrania/anencephaly in the first trimester.
- Associated anomalies such as omphalocele or spina bifida.
- Pitfalls: If the head is deep in the pelvis, the defect may be missed by transabdominal examination and thought to be a technical problem.
- Diagnosable from 10 to 14 weeks of gestation.
Associations: Associated anomalies, found in one-third of cases, such as spina bifida (most common), hydronephrosis, omphalocele, and cardiac defects. About 2% of fetuses with anencephaly have abnormal chromosomes, particularly trisomy 18.
Management: Termination of pregnancy should always be offered.
Prognosis: Uniformly lethal.
Recurrence risk: About 2-4%. Periconceptional folic acid supplementation can significantly decrease the recurrent risk. 50-70% of these defects can be prevented if a woman consumes sufficient folic acid daily before conception and throughout the first trimester of her pregnancy.