In a healthy person there are (diploid) 46 chromosomes. That the number of chromosomes are known 23 and its folds  is called oploid and not matched  to be born alive.

Numeric changes at one of the chromosomosomes is called monosomia, nullisomia nonoploid. Any augment is called trisomia, tetrasomia ( according the number of the increase of chromosome). But 13, 18, 21. Chromosomes have the third copy is directly related to life. An also any irregularities at sex chromosomes (excluding 45,Y) has the same risks..

Numeric chromosomal irregularities has low repetition risks because of the creation of de novo. But the couples should be offered prenatal diagnosis. Because some parents can carry this kind of irregularities at very small degrees within the somatic cells and they cannot be seen at routin checks. Besides the cell of trisomia can also be exist at gamete cells of one of the couples even if it is at low levels.