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Ultrasonography of the Fetal Subarachnoid Space

Department of Radiology, University of California Service, San Francisco General Hospital, San Francisco, California.

Address correspondence and reprint requests to Dr. Laing: Department of Radiology, Room 1X57, San Francisco General Hospital, 1001 Potrero Ave, San Francisco, CA 94110.

Abstract

A retrospective analysis of 122 asymptomatic pregnant women revealed the presence of a prominent echogenic line parallel to the inner table of the skull in 83 per cent of fetuses less than 30 weeks of age. A sonolucent space of 1 to 3 mm was noted between this line and the skull, suggesting an extra-axial fluid collection. After 30 weeks, this finding was noted in only 23 per cent of fetuses, and at term it was visualized in only 3 per cent of cases. This space, which is felt to represent the fetal subarachnoid space, should not be mistaken for an abnormal subarachnoid fluid collection.

Key Words: fetal skull anatomy • fetal subarachnoid space • pseudosubdural • obstetric ultrasound