Screening for trisomy 21 by maternal age fetal nuchal translucency thickness and maternal serum sample.

Aim: The aim of this paper was to examine the performance of two-stage first-trimester combined screening based on maternal age, fetal nuchal translucency (NT) thickness and maternal serum sample "free beta-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A)".

Methods: A combined screening for chromosomal anomalies was performed in 713 singleton pregnancies. We performed a two-stage screening with the blood taken at 8+0 to 10+6 weeks and the measurement of NT performed at 12+0 to 12+6 weeks. The maternal age related risk for trisomy 21 was calculated and adjusted according to the gestational age at the time of screening to derive the a-priori risk. The measured free beta-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A) were converted into a multiple of the median (MoM) for gestational age, adjusted for maternal weight, smoking status, ethnicity, method of conception (spontaneous or IVF) and parity. The measured NT was assessed in relationship of mesasure of CRL. Finally, the risk resulting by NT thickness and biochemical markers were multiplied by the a-priori risk to derive the patient-specific risk.

Results: The ultrascreen was considered positive in the case where the risk was greater than 1:250. In this case it was suggested the study of the fetal karyotype through an invasive test. In our study we had 23 positive cases after the combined screening: all patients have opted for the study of fetal karyotype, and in 5 cases the result was abnormal (trisomy 21). We had 1 case where the test was negative but the fetal karyotype was abnormal (trisomy 21). We have calculated sensitivity and false positive rate of the test.

Conclusion: In our study there were 707 cases with a normal karyotype or delivery of a phenotypically normal baby and 6 cases with trisomy 21. The detection rate of the first trimester screening for chromosomal anomalies was 83% with a false positive rate of 3,2%. The aim of the study was estimated the performance of two-step strategy screening. In our study, the performance of the screening model, based on the two-stage, was not higher than the performance of screening based on a single-step reported in literature. In our opinion, there is no potential advantage in terms of detection rate.