The cisterna magna (CM) is a large subarachnoid cerebrospinal space that lies between the caudal aspect ofthe cerebellum and the medulla oblongata, drainingthe fourth ventricle . The evaluation of the fetal cisterna magna is animportant part of obstetric ultrasound which targetsdetection of prenatal malformation of the fetal centralnervous system (CNS).Therefore, measuring the size ofthe cisterna magna has important clinical significancefor diagnosing fetal nervous system abnormalities.

Studies about the relationship between the anteroposterior diameter of the CM and the gestational age (GA) are still controversial. Some articles havepointed out that there was a good correlationbetween the diameter of the CM and GA

Prospective study that will include pregnant women who are between 20 weeks 0 days and 39 weeks 6 days of gestation. As in cross sectional study each fetus is considered only once. with use of magnetic resonance ultrasonography.( ultrasonography is a noninvasive imaging test uses high-frequency sound waves to create real-time pictures or video of internal organs or other soft tissues)

Inclusion Criteria:

• Pregnant women who are between 20 weeks 0 days and 39 weeks 6 days of gestation.
• Singleton pregnancy.

Exclusion Criteria:

.gestational age can not be confirmed.

• Fetal malformations or intrauterine growthretardation
• Women with chronic diseases.
• bodymass index >30

Prospective study that will include pregnant women who are between 20 weeks 0 days and 39 weeks 6 days of gestation. As in cross sectional study each fetus is considered only once. with use of magnetic resonance ultrasonography.( ultrasonography is a noninvasive imaging test uses high-frequency sound waves to create real-time pictures or video of internal organs or other soft tissues).