1.   Down syndrome is caused by a chromosome abnormality and results in retardation and characteristic physical abnormalities.

2.   It tests for chromosome abnormalities, including Down syndrome, and inherited diseases such as Tay-Sachs and cystic fibrosis.

3.   The inversion detected in our fetus is one of many such smaller defects, collectively known as chromosome structural abnormalities.

4.   Various registries, which keep track of chromosome abnormalities and their clinical implications, had been consulted, but he was not aware of any other case like ours.