Transvaginal sonographic measurement of cisterna magna in fetuses with abnormal karyotype.

The fetal cisterna magna was measured by transvaginal sonography in early pregnancy in 525 pregnant women at increased genetic risk in order to test whether the measurements thus obtained afforded a sonographic marker for abnormal fetal karyotype. The measurements of the cisterna magna increased as pregnancy progressed and showed a good linear correlation with gestational age (r = 0.616; p < 0.001). Chromosome abnormalities were found in 23 fetuses. In all these cases, the cisterna magna measurements were normal, and no differences were observed between fetuses with normal and abnormal karyotype in the different periods tested. While transvaginal sonographic measurement of the cisterna magna is easy to perform in early pregnancy, it is a poor screening test for the detection of abnormal fetal karyotypes.