P02.19: Prenatal diagnosis of Trisomy 21: we show a rare case of fetoplacental discrepancy

Abstract

Chorionic villus sampling (CVS) is the method of choice in pregnancies at risk for trisomies as a result of FTS for prenatal karyotyping before 15 weeks of pregnancy. The sensitivity of CVS is 99.6 percent for detection of nonlethal chromosomal aneuploidies and the risk of abortion is 0,5-1%. With the following case report we want to demonstrate a pitfall in the diagnosis of fetal aneuploidy from trophoblastic material. A 42-year-old primigravida at 13 weeks of gestation had an elevated risk for Trisomy 21 in combined first trimester test due to an abnormal maternal serum biochemistry (PAPP-A: 0,65 MoM, Beta-HCG: 8,18 MoM). The risk was 25 percent for Trisomy 21 in the fetus. A chorionic villus sampling was performed, with the diagnosis of a free Trisomy 21 in the direct preparation. A detailed ultrasound examination showed no no sonographic markers for Trisomy 21. Therefore in 14 + 0 an amniocentesis was carried out. The FISH test In 27 % of the amniotic cells in the FISH test displayed three signals for chromosome 21 indicating trisomy 21 or mosaic Trisomy 21, but the long term cultures from CVS and AC showed a normal female karyotype. The patient decided to continue the pregnancy, which is at 31 weeks now. Kalousek et al first described in 1997 that in approximately 2% of viable pregnancies studied by chorionic villus sampling at 9-11 weeks of gestation, the cytogenetic abnormality, most often trisomy, is confined to the placenta. The risk is twice as high using the direct method. So it is advisable to perform an amniocentesis to rule out a false positive test, when the fetus shows no pathologies at the ultrasound examination. Furthermore in non invasive prenatal tests evaluating placental free “cell free” (cf) nucleic acids in maternal blood an aneuploidy in placenta may cause a false positive result.