Bi-test (double test), nuchal translucency
Thanks to the great progress made in laboratory medicine and cytogenetics, it is possible during pregnancy, before birth, to estimate whether the foetus has malformations or chromosomal abnormalities and to decide whether to have more invasive tests (such as chorionic villus sampling or amniocentesis), preparing properly for what lies ahead. These tests are called prenatal screening tests and include the Bi-test and nuchal translucency. While chorionic villus sampling and amniocentesis provide a diagnosis of the actual risk of having a baby with malformations, they are invasive and increase the chance of losing the pregnancy (risk of miscarriage). Prenatal screening tests, on the other hand, are not invasive and pose no risk to the pregnancy, but they do not give certainty about the risk of having a baby with Down’s syndrome; they only give a probability. In practice, the result of the test is a number expressing a probability: if these tests show that the woman has a high risk of having a baby with Down’s syndrome, she can still choose to have prenatal diagnostic tests to be certain.
Why they are done
These tests are carried out on the advice of the gynaecologist to find out the probability of having a baby with chromosomal abnormalities. If the risk is high after these screening tests (double test, triple test and nuchal translucency), the woman can opt for the more invasive prenatal diagnostic tests (chorionic villus sampling and amniocentesis) to be certain, while if the result is favourable she may decide not to have them.
How they are done
- Bi-test: a blood test carried out between the 9th and 13th week of pregnancy, after an ultrasound check.
- Nuchal translucency: a special ultrasound scan carried out between the 11th and 13th week of pregnancy. If this test is combined with the double test, the detection rate approaches 90 per cent.
These tests do not require any preparation.
Results
- The advantage of the Bi-test over the triple test is that the blood sample can be taken earlier, in the very first weeks of pregnancy. The test measures two placental substances, the free beta fraction of chorionic gonadotrophin and pregnancy-associated plasma protein A (PAPP-A). These substances are present in the blood from the 8th-9th week of pregnancy, and low levels increase the risk that the baby has Down’s syndrome. The double test detects about 60-65 per cent of cases of Down’s syndrome.
- Nuchal translucency is the measurement of the thickness of the oedema (fluid build-up) under the skin at the back of the baby’s neck. This area tends to be larger in babies with serious malformations and chromosomal abnormalities. This test requires high-quality ultrasound equipment and specially trained medical staff to avoid misleading results, as it is not an ordinary ultrasound scan. This scan detects about 70-75 per cent of serious malformations, but combining and comparing nuchal translucency and the double test, carried out between the 11th and 13th week of pregnancy, increases the chance of detecting malformations and chromosomal abnormalities to about 90 per cent of cases.