The most common chromosomal disease is Down syndrome (trisomy 21). A person with Down syndrome can live a long life but needs constant assistance. The next frequent chromosomal diseases are trisomies 13 and 18, and Turner syndrome. The main cause for changes in the number and structure of chromosomes is an error in parental germ cells. That means that the sperm or egg does not contain the right number of chromosomes. Errors of this kind can arise due to the elevated age of the mother as well as many environmental factors (e.g., X-ray radiation, medications, household chemistry, alcohol). Also, random errors occur in nature and not all germ cells are always of high quality.
The risk of having a baby with a chromosomal disease increases with maternal age. For a 20-year-old woman, the risk of having a baby with Down syndrome is 1:2500, but for a 37-year-old it is already 1:100 (1%). For a 40-year-old woman, the risk is already 2% and for a 45-year-old 5% and more.
Nevertheless, most babies with Down syndrome are born to young mothers because the screening tests have not detected the disease. Also, some expecting mothers decide not to take part in screening tests. The argument “all my three children are well” may not hold for the fourth one, because maternal age is higher and the risks have gone up.
The risk of having a baby with Down syndrome depending on maternal age.
Of these chromosomal diseases that allow the baby to be born, the most common are trisomies in chromosomes 13, 18, 21 and monosomy X if a female baby has only one X chromosome instead of XX. Trisomy means that there are three 13, 18 or 21 chromosomes instead of the normal two. These three chromosomes are relatively small in the genome and contain a limited number of genes necessary for foetal development. This is why a baby with trisomy can be born at all. These three trisomies make up more than 70% of all chromosomal diseases and they can be detected easily in maternal blood via NIPTIFY analysis. The data on the graph on the left is from a scientific study.