Normal human somatic cells include 46 chromosomes. If the number chromosomes is 23 and its multiples, then it cannot be verified with normal birth and this numeric value refers to euploidy.
Any change in the number of a single chromosome is called as aneuploidy. As the deficient number of chromosome is called as monosomy or nullisomy, extra number of chromosomes is called as trisomy or tetrasomy. Any trisomies occurring in the autosomal chromosomes can be detrimental except at chromosomes 13, 18 and 21. Numerical anomalies in sex chromosomes – except for 45,Y – are directly related to survival.
Risk of recurrence of numerical chromosomal abnormalities is usually low r due to their de novo property. However, it is still important for the couples to take prenatal tests for every new pregnancy, because some parents may carry such chromosomal abnormalities at a very low rate in their somatic cells. In some families, mosaicism lymphocyte cells might be very low and this fact may not be realised and recognised during a routine investigation. Furthermore, trisomic cells might be observed at different ratios in maternal or paternal gametes.