Berlin (dpa) - If the likelihood of a Down syndrome in the unborn child is calculated, blood tests are considered relatively reliable.

In the blood of the mother-to-be, apart from her own DNA, there are also fragments of the child's genome, which can be used to determine the likelihood of a trisomy 21 (Down's syndrome). People with Down syndrome have one more chromosome in each cell than others, that is, 47 instead of 46. Chromosome 21 is triplicate rather than twofold, as in other humans, hence trisomy 21.

Blood tests are possible from the tenth week of pregnancy on and can predict with high probability, whether a trisomy 21 is present. However, they do not allow for a conclusive diagnosis. Therefore, further tests are required to confirm the result - such as an amniocentesis or a placenta puncture, which are associated with the risk of miscarriage. According to the Federal Center for Health Education, it is 0.5 to 2 percent.

In very rare cases, the blood test also contains false-negative findings. Rarer forms such as trisomy 13 (P├Ątau syndrome) or trisomy 18 (Edwards syndrome) can not reliably detect the so-called non-invasive prenatal diagnosis.

Episode of the first described by the English physician John Langdon Down in 1866 syndrome are about physical abnormalities in size, weight and head shape. Added to this are slower motor, mental and linguistic development as well as often organic damage.

About one in every 800 children is born with Down syndrome, the probability increases with the age of the mother. Around five million people worldwide live with it.