Abnormalities of fetal cranial contour in sonographic detection of spina bifida: evaluation of the "lemon" sign.

Previous reports have shown that visualization of the "lemon" sign on fetal cranial sonograms may indicate the presence of spina bifida before 24 weeks. To further characterize this sign and determine whether other factors might be associated with it, the authors retrospectively (n = 31) and prospectively (n = 19) evaluated 50 fetuses with spina bifida. The lemon sign was noted in 32 (64%), including 24 of 27 (89%) fetuses evaluated before 24 weeks. The presence of both ventricular dilatation, found in 41 (82%) of the fetuses, and a lemon sign correctly predicted spina bifida in all but one case. The location of the spinal defect and other anomalies were not associated with the lemon sign. The lemon sign is an important indicator of spina bifida, particularly in high-risk pregnancies, and should improve sonographic detection of spina bifida before 24 weeks.