First trimester screening: the BUN study

First trimester risk assessment for trisomies 21 and 18 is rapidly transitioning from an investigational procedure performed at a few major centers to the clinical arena. The BUN study (Biochemistry, Ultrasound, Nuchal translucency) was conceived to evaluate the performance of first trimester screening using PAPP-A, free beta HCG, and ultrasound measurement of the nuchal translucency when introduced into practice. Over a 4-year period, 13 prenatal diagnostic centers evaluated over 8500 patients and reported an 85.2% trisomy 21 detection rate with a 9.4% false positive rate. Further evaluation of the data revealed that, once training and experience were accomplished, sonographers could perform NT measurements consistent with reported standards. In approximately half of the patients enrolled in the study, second trimester serum analytes were obtained leading to additional trisomy 21 detection but with an unacceptably high false positive rate. A "sequential contingency" screening approach using both first and second trimester values in some patients may provide the additional detection afforded by measuring second trimester analytes with a limited invasive procedure rate.