© 2015 by the American Institute of Ultrasound in Medicine. The purpose of this series was to describe sonographic features of an isolated widened fetal subarachnoid space with a thin cerebral mantle and possible associations. Between January 2004 and December 2013, fetuses with a prenatal diagnosis of a widened subarachnoid space were prospectively recruited and followed. Histories of medical and familial diseases, as well as other demographic data such as drug exposure and lifestyles, were assessed and prospectively recorded. The women were investigated for possible associated factors. Ten pregnant women were recruited. Their fetuses showed various degrees of a widened subarachnoid space, ranging from 5 to 20 mm. Nearly all were diagnosed in the second half of pregnancy. Four cases had normal brain structures documented at midpregnancy anomaly screening. Only 1 case had a prenatal diagnosis of a widened subarachnoid space at 20 weeks' gestation. Two fetuses had exposure to alcohol in utero; 2 were proven to have cytomegalovirus infection; 1 had subarachnoid hemorrhage secondary to maternal use of warfarin; and 1 had a diagnosis of lissencephaly. Only 1 case in this series had normal postnatal development. A prenatal series of fetal widened subarachnoid spaces with possible associated factors is described. Although such relationships were not fully proven, they should be index cases for future studies.