(A) Sagittal view of a normal fetal head and posterior fossa at 12+4 weeks of gestation. In the normal fetus, the IT is a fluid-filled area (the fourth ventricle) in the normal posterior brain in the mid-sagittal plane at the time of a detailed anatomic survey in the late first trimester.
(B) Sagittal view of a fetal head at 12+6 weeks of gestation with lack of visualization IT. This has been described as an early marker for open spina bifida.