Edward's Syndrome occurs when there are three chromosomes in the eighteenth position instead of the normal two. For this reason, it is also called Trisomy 18. It is often suspected from ultrasound or characteristics seen at birth, but it is diagnosed using a karyotype. For more information about Edward's Syndrome, including diagnosis and characteristics, see Edward's Syndrome Resources
Before birth, a sample of the placenta is taken during chorionic villus sampling or, most commonly, during an amniocentesis. From these samples, cells are grown which are then analyzed for the number of chromosomes. After birth, the baby's blood may be analyzed in this way to verify a suspected diagnosis.
Here is a karyotype showing Edward's Syndrome (Trisomy 18). Note the three chromosomes in the 18th position.
The 47,XX,+18 indicates that there are 47 chromosomes (normal is 46), that it is a girl (Edward's Syndrome affects girls three times as often as boys), and that the extra chromosome is number 18.
Karyotype results are usually reported by saying how many cells were tested and how many found the additional chromosome. In the mosaic form of Edward's Syndrome, the extra chromosome is not in every cell. Likewise, in partial Edward's Syndrome, part of an additional chromosome is found.
For information about diagnosis of, characteristics of, and stories about Edward's Syndrome, see Edward's Syndrome Resources