Comparison of First Trimester, Second Trimesterand Integrated Downs Syndrome ScreeningResults in Unaffected Pregnancies

Our aim was to compare the results of first trimester combined test, second trimester triple test, and integrated test in the same pregnant population. We retrospectively studied 927 women, all giving birth to an unaffected baby except for two cases of Down's syndrome. The women underwent a nuchal translucency ultrasound measurement and a blood sampling for pregnancy-associated plasma protein A and free β-hCG subunit (free total chorionic gonadotropin subunit) assay in the first trimester of pregnancy. A second trimester biochemical screening (α-fetoprotein, unconjugated oestriol and total hCG) was performed later. The correlations between each pair of markers and between each marker level and maternal age were calculated. No marker showed significant correlation with any other or with maternal age, with the obvious exception of free β-hCG subunit and total hCG. The false-positive rate (cut-off level: 1 in 350 at term) was 1.5% for the first trimester test, 3.6% for the second trimester test and 0.54% for the integrated test. In 10/14 pregnancies, the increased risk in the first trimester was not confirmed neither in the second trimester nor by the integrated test. In 29/33 women with an increased risk in the second trimester, the first trimester and the integrated test results were discordant. The absence of correlation among different marker levels suggests that the information supplied by the first and second trimester tests is different. Integrating first and second trimester markers in a single test could pose the ethical problem of withholding first trimester results and thus denying the possible advantages of an earlier pregnancy termination.