The aim of this study is to determine the effect of imaging angle on the measurement of fetal cisterna magna (CM) in the second trimester.

Fetal CM will be prospectively measured in women at 18-24.6 weeks' gestation. Measurements will be made on different angles between the transducer and fetal cerebellar axial plane with internal landmarks.

Design: Observative, cross sectional

Patients and methods: women with low risk single pregnancy will be studied. Fetuses with multiple pregnancies, maternal chronic disease and severe congenital anomalies except mega cisterna magna (MCM) will be excluded from the study. Measurements will be performed on the axial cerebellar plane.The anteroposterior measurement will be made between the posterior border of the cerebellar vermis and the internal face of the occipital bone.

The primary outcome variables will be cisterna magna length measured on different angles between the transducer and the axial cerebellar plane of the fetal head.

Cisterna magna length measurement related to fetal presentation and gender will be secondary outcomes.

Inclusion Criteria:

• Pregnant women between the 18th and 24th weeks.
• Confirmed gestational age by regular menstrual cycle or determined from crown- rump length at first trimester ultrasonography for those with unknown menstruation date .

Exclusion Criteria:

• Uncertain gestational age
• Twin or multiple pregnancy.
• Increased risk for aneuploidy determined by first trimester screening.
• Absence of intracranial translucency in first trimester ultrasound examination.
• Fetal malformations identified EXCEPT megacisterna magna.
• Karyotype anomaly.
• Chronic maternal diseases such as arterial hypertension, diabetes mellitus, hypothyroidism and hyperthyroidism.
• Failure to obtain accurate measurement for technical reasons.

Women with low risk pregnancy between the 18th and 24th weeks with confirm gestational age who wil be referred to our prenatal diagnosis unit for ultrasonography to be performed in their second trimester.