The doctor ordered blood from the parents to run tests on. Each set is numbered. Some fetuses develop with 47 chromosomes, which means that instead of all pairs, they have one set of three, a condition called trisomy. Trisomies are the most common type of chromosomal abnormality. The risk of having a baby with trisomy increases with age, which is why healthcare providers suggest genetic tests for women 35 and older.
Doctors usually use blood tests to determine the cause of the abnormality.
Though most trisomies are a result of biological errors during the early stages of cell division, sometimes the causes are genetic. Even parents with no outward signs of a defect can still pass on an extra chromosome. Sometimes parents choose not to have biological children if they know one or both of them has an abnormal chromosomal makeup. They need to carefully discuss all the issues with their doctor or a geneticist before deciding whether to continue the pregnancy. While all babies with Down syndrome have some degree of mental retardation, it is severe in only about 10 percent.