Chance a chromosome abnormality will occur again?

The chance for a chromosome abnormality to happen in a subsequent pregnancy depends upon the type of chromosome abnormality. In general, the chance for a numerical chromosome abnormality (where the number of chromosomes is more or less than 46 total in each cell) to happen again, is 1 percent or less. This means there is a 99 percent chance a chromosome problem will not happen again in a future pregnancy.

The chance for a structural chromosome abnormality to happen again depends upon whether the structural chromosome abnormality was inherited from a parent. If one of the parents has the same structural chromosome problem, then there is an increased chance (up to 100 percent, depending upon the chromosome finding) for the chromosome abnormality to be seen in a future pregnancy.

If the structural chromosome abnormality is not present in one of the parents, but is only seen in the child born with the abnormality, the chance for the structural chromosome abnormality to be seen again in a future pregnancy is usually very low (less than 1 percent). Since the chromosome problem was not inherited, it is sometimes called de novo (meaning new).

Rarely, a de novo structural chromosome problem is seen more than once in a family. The explanation for this is that one of the parents has a mixture of cell types in their eggs (if it is the mother) or the sperm (if it is the father) only. In other words, although the chromosome study performed from a blood sample on the parent was normal, the germ cells (eggs and sperm) are actually a mixture of some normal cells and some cells with the structural chromosome abnormality. This is also known as "germline mosaicism".

When a child is born with a mosaic chromosome abnormality, the chance for a mosaic chromosome problem to happen again depends upon how the mosaicism originated. If the mosaicism resulted from a cell division error in both meiosis and mitosis, the chance is 1 percent for the same problem to reoccur in a future pregnancy. If the mosaicism resulted from a cell division error after fertilization, the chance for recurrence is even lower (less than 1 percent).