Chromosome abnormalities are defined as anything that is different from the normal compliment of chromosomes.  Whilst there are many alterations to the normal chromosome compliment, that go unnoticed and cause no effects whatsoever, there are many abnormalities that do cause serious abnormalities, some that a person can live with and some that are incompatible with life. It is hard to estimate the number of chromosomal abnormalities as many may end in miscarriage without ever being identified as a chromosomal anomaly.

Some of the conditions mentioned in the Pregnancy Complications are caused through chromosomal abnormalities and go into more detail. If you want to read more detail about chromosome abnormalities follow the link to Wikipedia