Generality
The bi test is a screening test that is performed in the first trimester of pregnancy.
This investigation associates the measurement of fetal nuchal translucency (by ultrasound) with the measurement of two substances present in the maternal blood:
- A protein linked to pregnancy: PAPP-A (plasma protein A associated with pregnancy);
- A hormone produced by the embryo: hCG (chorionic gonadotropin).
The bi test provides an estimate of the risk of chromosomal abnormalities and congenital malformations in the fetus.
What's this
This survey involves two different approaches:
- A "maternal abdominal ultrasound to calculate nuchal translucency (translucent slit, due to a small amount of fluid under the skin of the nape of the fetus, whose thickness increase is related to Down syndrome);
- A blood draw from the expectant mother to assess PAPP-A and hCG levels.
The bi test allows to evaluate the risk that a fetus may be a carrier of some chromosome alterations such as, for example, trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome), already in the first trimester of gestation.
The examination is not invasive, therefore it does not involve any danger to the baby or risk of abortion and can be requested by all pregnant women.
Because it is measured
The bi test is performed to calculate - without certainty, but with very high approximation - the risk of a fetus to be a carrier of chromosomal abnormalities and / or to develop congenital malformations, such as, for example, trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome).
This examination is usually performed between the eleventh and fourteenth week of pregnancy, when the fetus is between 45 and 84 mm long.
- The first part of the bi test is based on the taking of a small sample of maternal blood, in which two substances of placental origin are measured, which can be altered in pregnancies with a fetus affected by Down syndrome. These substances are respectively called β-hCG (free beta fraction of chorionic gonadotropin) and PAPP-A (plasma protein A associated with pregnancy).
- The second part of the bi test uses, instead, the ultrasound measurement of nuchal translucency.
The combined execution and evaluation of these two tests allows the formulation of a risk in probabilistic terms that is rather sensitive and specific.
However, if the bi test provides worrying results, further diagnostic investigations (eg amniocentesis or chorionic villus analysis) may be recommended.
