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Anencephaly

Frank H Miller, MD
Faye C Laing, MD

June 8, 1994

Presentation

In this case from 1991, a 19-week fetus with elevated alpha-fetoprotein is scanned for amniocentesis.

Imaging Findings

Ultrasound Examination

Ultrasound examination indicates that the cranium and brain are not formed. The calvarium is not visible above the orbits. Moderate polyhydramnios is apparent.

Diagnosis

Anencephaly

Discussion

After 12-13 gestational weeks, the normal fetal brain should be readily visible on transabdominal sonographic examination. Anencephaly will result if there is failure of complete fusion of the neural tube. This is the most common and most severe form of open neural tube defect and has a 5% rate of recurrence in subsequent pregnancies.

Ultrasound findings include absence of the cranium cephalad to the orbits and bulging eyes ("frog-like" appearance). Polyhydramnios, which is attributed to insufficient fetal swallowing, occurs in 40-50% of cases after 26 weeks gestational age.

References

1. Filly RA. Ultrasound evaluation of the fetal neural axis. In: Callen P, editor. Ultrasonography in obstetrics and gynecology. 3rd ed. Philadelphia: Saunders, 1994:219-21.

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