Abstract
Fetal neural tube defects are the second most common serious fetal birth defects.
In fetuses with open spina bifida there is leakage of cerebrospinal fluid into the
amniotic cavity which causes hypertension in the subarachnoid space leading to caudal
displacement of the brain. The cranial signs (lemon and banana) are detectable during
the second trimester scan. In the first trimester, there are alterations in the appearance
of the posterior brain at 11–13 weeks. In fetuses with open spina bifida, the posterior
shift of the brain is evident from the first trimester, resulting in compression of
the fourth ventricle and alteration of the normal intracranial translucency. In the
midsagittal view of the face, if the fourth ventricle is not visible, it should arouse
the suspicion of the possibility of an underlying open spina bifida and hence, a detailed
examination of the fetal spine should be performed. Also, because of the posterior
shift of the brain, the brainstem (BS) appears relatively enlarged and shifted backwards.
The BS to brainstem to occipital bone distance (BSOB) [BS/BSOB] ratio is increased
and it is usually more than one. At 11–13-week scan, the midsagittal view of the face
is the standard view obtained in every fetus and it is feasible to visualize the posterior
region of brain in this view. Suspicious cases should undergo a detailed assessment
of the spine, especially on transvaginal scan followed by targeted examination of
the spine at 16–18 weeks.
Keywords Neural tube defects - Posterior fossa - Intracranial translucency - Brainstem to occipital
bone distance - Transvaginal scan
