Detecting open spina bifida at the 11–13‐week scan by assessing intracranial translucency and the posterior brain region: mid‐sagittal or axial plane?

Prenatal diagnosis of open spina bifida in the second trimester can be achieved by identification of indirect cranial and cerebellar signs (lemon and banana signs), rather than by direct examination of the spine to locate the lesion1. Screening at 11–13 weeks’ gestation is now performed not only for the measurement of nuchal translucency (NT), but also for detecting severe malformations and identifying pregnancies at high risk for adverse fetal and maternal outcome2. In the first trimester, open spina bifida has not been amenable to diagnosis because the banana and lemon signs cannot be relied upon at this gestational age3,4. However, we have recently reported that fetuses with open spina bifida have easily detectable abnormalities in the posterior fossa of the brain, which could be incorporated into the routine 11–13-week fetal sonographic assessment5,6. Our observation has been confirmed by several retrospective and prospective studies, many of which are reported in this issue of the Journal7–15. In our original report, examination of the posterior fossa was undertaken in the mid-sagittal plane that is used routinely for measurement of fetal NT and assessment of the nasal bone5. However, some studies have suggested that more reliable assessment of the posterior fossa and diagnosis of open spina bifida can be achieved using axial planes of the fetal head9,16–18. In this Opinion we discuss whether assessment of the posterior brain area should be undertaken in the mid-sagittal or axial plane.