BIOMETRY OF THE FETAL CISTERNA MAGNA - ESTIMATES OF THE ABILITY TO DETECT TRISOMY-18

Measurements of the fetal cisterna magna were obtained from a prospect ive sample of patients undergoing diagnostic obstetric ultrasound exam inations. These normal measurements were then compared to a retrospect ive sample of ultrasound scans from fetuses with the diagnosis of tris omy 18 by amniocentesis. The fetal cisterna magna increases in size th roughout pregnancy. The sample from fetuses with trisomy 18 was differ ent and had a higher rare of small and large cisterna magnas. After th e two samples, were mixed it was determined that a cut-off of 2.5 stan dard deviations for the fetal cisterna magna would give a sensitivity of 26.3%, specificity of 99.1%, positive predictive value of 50.0% and negative predictive value of 97.6% for trisomy 18. The probability th at a fetal cisterna magna of abnormal size will predict trisomy 18 in an early second-trimester ultrasound examination with the 2.5 standard deviation cut-off was estimated at 0.019. A fetal cisterna magna that is of abnormal size appears to be of diagnostic value in identifying the fetus with trisomy 18, but its sensitivity is low.