Evaluation of prenatal optic chiasm (OC) is important for the differential diagnosis of septo-optic dysplasia (SOD), which is a congenital optic disc anomaly, and the absence of the cavum septum pellucidum (CSP). Septo-optic dysplasia is associated with a wide range of neurological abnormalities, including hypoplasia of the visual pathways, CSP agenesis, and developmental delay.

Up to now, optic chiasm measurement were all done in the third trimester. in the present study, OC will be measure directly in a 2D ultrasound in coronal plane within the second trimester. In fetuses with SP agenesis, the morphology and width of the OC visual pathway may be a suitable tool to assess its development. It can also assist in the difficult task of providing prenatal counseling when faced with a diagnosis of CP agenesis. therefore In the present study, it was planned to show the optic chiasm in the screening of septooptic dysplasia by ultrasound between 16-20 weeks of gestation, and to measure the optic chiasma width and length, and to determine the mean values in the second trimester.

Inclusion Criteria:

• aged greater than 18 years old
• having singleton pregnancy

Exclusion Criteria:

• being under the age of 18
• having IVF pregnancy
• having multiple pregnancy
• comorbidity during pregnancy
• smoking history
• presence of familial genetic disease