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The optic chiasm (OC) is an X-shaped structure formed by the crossing of the optic nerves in an axial view of the fetal brain. It is located in the chiasmatic cistern and is surrounded by the Willis circle. The anterior cerebral arteries lay anteriorly, the posterior communicant arteries lay laterally, and the posterior cerebral arteries are identified posteriorly. The main advantage of this technique is the accessibility to all sonographers, no need of TV ultrasound and it could be performed from 20 weeks onwards. The operator has to obtain an axial image of the fetal head at the level of the transventricular plane as described by ISUOG guidelines. Slide the probe caudally and then activate color Doppler to identify the Willis circle (only for orientation; WMF low, PRF 1.3-2.4 kHz). Three steps are required from this point to achieve the adequate image of the OC and perform the width measurement: 1. Rotate the transducer in the frontal part until at least one orbit is seen (the anterior orbit and cerebral peduncles must always be identified). The rotating angle is 20-40º and varies according to the gestational age. 2. Identify the OC by its characteristic echogenic X shape. The activation of the color Doppler shows the middle cerebral arteries (MCA) arising from internal carotids on the sides. 3. Measure the width of the OC in the middle of the X shape with callipers on to on. We recommend performing the measurement switching the color Doppler off and turning the gain down when needed for better delimitation of the borders. This three-step technique for fetal OC assessment is easy to perform and does not increase significantly the scanning time. [ABSTRACT FROM AUTHOR]