Screening for Down Syndrome
Down syndrome is the most common genetic disorder with leading symptoms such as anatomical abnormalities that expand to several organ systems and varying degrees of intellectual disability. It is caused by the faulty fission of chromosome 21.
The incidence of the syndrome is 1.17 thousandth in Hungary, which means one case in every 1000 pregnancies. The risk increases with the age of the mother, thus we recommend chromosome screening as part of prenatal care for women over the age of 35. Naturally, screening can be requested by younger mothers-to-be as well.
There are several screening methods to choose from: during a fetal chromosome examination, a sample is taken from the placenta (chorion biopsy) or from the amniotic fluid (amniocentesis) for laboratory testing. However, as sampling is done through the abdominal wall and the process carries a 1% risk of miscarriage, women often choose other types of screening.
The so called combined test is performed between weeks 12-14 of the pregnancy by an ultrasound examination and a blood test. The screening method is 95% sure, and aside from Down syndrome, it can reveal the presence of Edwards and Patau syndromes as well.
The third possibility is to request an integrated test that compares the results of two blood tests and the ultrasound images. The first blood sample is drawn in the first trimester during weeks 12-13, while the second sampling takes place in the second trimester between weeks 15-18. It shows Down syndrome and spina bifida with a 90-95% accuracy.
The newest and most effective method is sequential screening. This means that women who had a combined test between weeks 11-14 are divided into three groups based on their results. The first group consists of mothers-to-be who have fetuses with no symptoms: they only need to take an AFP test at week 16 and a genetic ultrasound examination between weeks 18-20. In cases of the second group the possibility of the syndrome cannot be excluded, but the results are not as worrying as to expose them to the risk of amniocentesis. For them, we recommend another blood test at week 16 and a detailed ultrasound examination between weeks 18-20. The third group had positive results on the combined test, which has to be confirmed by an invasive method (amniocentesis or chorion biopsy). Usually only in 1-2% of women need further invasive tests based on the results.