Measurements of the fetal cisterna magna were obtained from a prospective sample of patients undergoing diagnostic obstetric ultrasound examinations. These normal measurements were then compared to a retrospective sample of ultrasound scans from fetuses with the diagnosis of trisomy 18 by amniocentesis. The fetal cisterna magna increases in size throughout pregnancy. The sample from fetuses with trisomy 18 was different and had a higher rate of small and large cisterna magnas. After the two samples were mixed, it was determined that a cut-off of 2.5 standard 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 that 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.