Definition
Trisomy 18, also known as Edwards syndrome, is a serious genetic mutation. In this case, chromosome 18 occurs three times instead of the usual two times in the body cells. After trisomy 21, also called Down syndrome, trisomy 18 is the second most common: on average, about 1 in 6000 births is affected. Edwards syndrome is a syndrome with severe symptoms and a very poor prognosis. About 90% of affected fetuses die before birth.
Causes
Trisomy 18 is a so-called chromosomal aberration, in which chromosome 18 is present in the body cells in a defective form. Normally, each somatic cell has 23 pairs of chromosomes with two chromosomes each – also called double set of chromosomes. One of the chromosomes comes from the father, one from the mother.
This is due to the fact that the egg cell and the sperm cell each have only one so-called half chromosome set. When the two cells fuse together, each of the 23 chromosomes is duplicated. There are several possibilities at which point in this process errors occur, which lead to a third 18th chromosome: For example, already during the development of the maternal egg cell, an incorrect division of the chromosomes may have occurred.
However, it is also conceivable that an error occurred in the development of the father’s sperm cells. However, the error can also occur only after the fusion of sperm and egg in further cell divisions. Depending on when the error leading to trisomy occurred, there are 3 different forms of trisomy 18: free trisomy, mosaic trisomy and translocation trisomy.
The probability of trisomy 18 increases with the age of the mother at pregnancy. Nevertheless, young mothers can also have children with trisomy 18. The age of the mother is a risk factor and should not be categorically associated with trisomy.
Diagnosis
If the treating gynaecologist notices typical irregularities in the unborn fetus during an ultrasound examination, these changes may indicate trisomy 18. Subsequently, a blood test can be performed on the mother. If certain blood values of the mother are elevated, this may indicate an increased probability of chromosomal aberration.
Recently, a blood test of the mother can also be used for a concrete analysis for trisomies, because fragments of fetal DNA can also be found in the mother’s blood. There are three tests available: the panorama test, the harmony test or the prenatal test. Nevertheless, it must be said that a trisomy detection based on the mother’s blood does not give a 100% certainty.
The test can be both false negative and false positive. In order to make a reliable diagnosis, invasive examinations are therefore necessary. However, the risks of these examinations should not be underestimated and can, in the worst case, lead to miscarriage. A tissue sample of the placenta or the amniotic fluid is taken in order to examine the fetal cells contained therein in the laboratory for a possible trisomy.
