ad. 3. What is a genetic scan?
The term “genetic” ultrasound is often overused, since people tend to use this word referring to every “more precise” ultrasound examination during pregnancy. However, it is defined as an element of the second trimester anomaly scan which, on the basis of a series of ultrasound markers, qualifies a given case for the amniocentesis or not. It is performed between completed 16 weeks and 20 weeks of gestation by a specialist experienced in detecting the Down syndrome (trisomy 21). Nowadays, there are a few protocols of genetic ultrasound. At our centre, we apply two of them: combined Nyberg 2001 and Bromley 2002 as well as DeVore 2000. These protocols are updated with constantly released reports. The efficiency of such an examination ranges between 75% and 91%, depending on applied protocol.
The technique of the “genetic” risk assessment (so called genetic scan) was developed in the United States earlier than the European technique of screening tests in the first trimester. It is constantly developed thanks to the experience of sonologists all over the world and it consists in detecting features of fetal appearance, which might indicate the Down syndrome. There are only a few highly qualified centres in the USA, which have genetic scan in their offer. This kind of examination is not performed in Europe nearly at all because of the “tradition” of 11 – 13+6 screening tests.
Genetic scan is recommended to:
- the patients who, in the first trimester of pregnancy (11+0-13+6 weeks) did not undergo a screening test for the Down syndrome and other genetic syndromes
- the patients, who after a screening test in the first trimester of pregnancy were in a group of an increased risk and refused to undergo offered diagnostic tests, e.g. amniocentesis or chorionic villus sampling.
Why is experience so important in genetic ultrasound?
Detecting the Down syndrome in the second trimester might be very difficult. Not all children with trisomy 21 show anatomical defects, not all of them suffer from problems with the ossification of nasal bone or increased nuchal fold thickness. A lot of fetuses with the Down syndrome look almost identical as fetuses without anomalies. For this reason, it is highly recommended that the examination shall be performed by a specialist, who individually detected at least 20 cases of trisomy 21 in the second trimester (which is very difficult because the average occurrence of the Down syndrome is 1/1000 cases) and applies genetic ultrasound algorithms on a daily basis. Nevertheless, such a specialist should keep in mind that this method is not 100% effective and it will not detect all cases of the Down syndrome.
During an ultrasound scan chromosomes are not visible, but with a proper technique defects and certain features of a baby that are the signs of possible genetic disorders can be recognized. However, in each case it is essential to conduct a conclusive test such as amniocentesis, chorionic villus sampling or fetal blood sampling.
Neither the examination at 12 weeks not the one in the third trimester at 30 weeks can be called a genetic scan!