Second-trimester prenatal screening for trisomy 21 using biochemical markers: a 7-year experience in one cytogenetic laboratory.

Background Screening for trisomy 21 in the second trimester of pregnancy using biochemical markers is an established part of prenatal care in many developed countries. Objective The present study was aimed at determining the incidence of trisomy 21 and other chromosomal abnormalities in women undergoing prenatal chromosome analysis after a second-trimester biochemical screening. Results A total of 2832 karyotypes were analyzed following a positive second-trimester maternal serum screening (risk ≥ 1/250) between 1998 and 2004. Thirty-nine cases of trisomy 21 and 40 other chromosomal abnormalities were detected. The positive predictive value was 1 in 73 karyotypes for trisomy 21 and 1 in 71 for the other chromosomal abnormalities. However, a temporal decline in the dection rate of trisomy 21 was noted, from 1/63 in 1998 to 1/221 in 2004. This change was attributable to an increasing number of pregnant women having first-trimester ultrasound nuchal translucency measurement. Conclusion Given the fact that nuchal translucency measurement combined with first-trimester biochemical marker screening has a positive predictive value of trisomy 21 comparable to that obtained following second-trimester biochemical screening, we should question whether to move trisomy 21 screening by maternal serum markers from the second trimester to the first trimester in conjunction with nuchal translucency measurement. Furthermore, genetic counseling prior to the amniocentesis should discuss the relatively high probability that a chromosomal abnormality other than trisomy 21 may be identified. Copyright © 2006 John Wiley & Sons, Ltd.