Sonography of anencephaly: pitfalls in early diagnosis

A case in which the diagnosis of anencephaly was missed on a standard obstetrical sonogram performed at 12.5 menstrual weeks is described. Prompted by this case, we retrospectively reviewed the appearance of the cephalic pole of normal fetuses in 47 pregnancies ranging in menstrual age from 11 weeks to 16 weeks. Results indicate that the abnormal cephalic pole in the index case was similar to the appearance of many normal fetuses of similar menstrual age. We conclude that while anencephaly may be diagnosed as early as 11 menstrual weeks to 12 menstrual weeks, this defect may be difficult to diagnose on sonograms using standard sonographic techniques prior to 14 menstrual weeks.