Prospective validation of first-trimester combined screening for trisomy 21.

OBJECTIVE: To examine the performance of the new algorithm in screening for trisomy 21 by a combination of maternal age, fetal nuchal translucency (NT) and maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A).

METHODS: This was a prospective screening study for trisomy 21 in singleton pregnancies at 11 + 0 to 13 + 6 weeks of gestation using an algorithm combining maternal age, fetal NT thickness based on the mixture model for the assessment of NT, and maternal serum free beta-hCG and PAPP-A based on a multiple regression model for the assessment of serum biochemistry. The NT measurements were performed by 60 operators who had obtained The Fetal Medicine Foundation certificate of competence in the 11-13-week scan.

RESULTS: The study population consisted of 19 614 pregnancies with a normal karyotype or delivery of a phenotypically normal baby (euploid group) and 122 cases of trisomy 21. In the euploid fetuses the NT was above the previously defined 50(th), 95(th) and 99(th) centiles in 10 033 (51.2%), 618 (3.2%) and 123 (0.6%) cases and the respective values for trisomy 21 were 117 (95.9%), 94 (77.0%) and 57 (46.7%). The median fetal NT was within 0.1 mm of the expected in 47 (78.3%) of the 60 sonographers and within 0.2 mm in all. In the euploid fetuses the median free beta-hCG was 1.0 (range, 0.1-29.4) multiples of the median (MoM) and the median PAPP-A was 1.0 (range, 0.2-3.3) MoM. The median MoM values were 1.0 or close to 1.0 MoM for each subgroup of pregnancy characteristics, including gestations of 11, 12 and 13 weeks, maternal weight of < 60 kg, 60-80 kg and > 80 kg, different ethnic origins, cigarette smokers and non-smokers, natural conception and in vitro fertilization. For a false-positive rate of 3%, the detection rate of trisomy 21 in screening by maternal age and fetal NT was 81% (95% CI, 73-89%), by maternal age and maternal serum biochemistry it was 63% (95% CI, 56-72%) and by combined screening based on maternal age, fetal NT and maternal serum biochemistry it was 90% (95% CI, 84-96%).

CONCLUSION: This study has validated the new risk algorithm and demonstrated that in combined screening for trisomy 21 based on maternal age, fetal NT and free beta-hCG and PAPP-A the detection rate is about 90% for a 3% false-positive rate.