A biochemical screening test is a hormonal dosage in maternal blood of the free fraction of chorionic gonadotropin (beta-HCG) and of plasmatic protein A (PAPP-A), produced by the placenta, that may change in the presence of a pathological karyotype.
• Beta - HCG
The beta HCG is present in the maternal blood from the pregnancy beginning and the concentration decreases with the gestational development. If a fetus has a trisomy 21 (Down syndrome) the levels of beta-HCG are higher than a normal fetus. Higher is a level of beta-HCG, greater is the risk of trisomy 21. For the trisomies 13 and 18 the value are decreased; for sexual chromosomes aneuploidies the levels are normal.
• PAPP - A
• The PAPP-A quantity in the maternal blood generally increase with the pregnancy development.
If a fetus has the 13, 18, 21 trisomies or the numerical alterations of sexual chromosomes, hormonal values are lower than those for fetus with a normal karyotype. Lower is the value of PAPP-A and greater is the risk of fetal chromosomal abnormality.
Beta – HCG and PAPP-A can be associated together with the ultrasound evaluation of the nuchal translucency of the fetus and with a maternal age (combined test) to provide a personal risk (reliability upper 90%) having a fetus with the chromosomal pathology, and then depending on risk, deciding if make the prenatal diagnosis throughout chorionic vullus sampling or amniocentesis.
When and how it is performed?
It’s made a maternal blood withdrawal between the 11^+0 and 13^+6 pregnancy weeks. Fasting is not required.
Within 7 days from draw blood.