Trisomy 18 is a genetic condition related to the presence of an extra chromosome 18 caused by a problem that occurs when cells divide in the egg, sperm, or fertilized egg. The extra chromosome causes the fetus to develop abnormally with a number of physical and mental problems.

Trisomy 18 is also called Edward's syndrome. It is the second most common trisomy condition. (Down syndrome is the most common.) A fetus with trisomy 18 has three copies of chromosome 18 in each cell.

Trisomy 18 can be identified during pregnancy. Doctors can do prenatal tests and fetal ultrasounds to screen for problems, and they can do chromosome tests to diagnose trisomy 18.

Many fetuses with trisomy 18 do not survive to birth, but some are born and live a couple of months to a couple of years. Babies born with trisomy 18 can have heart and kidney problems, a small head with low-set ears, a chest with an unusual shape, and crossed legs. They also have severe intellectual disability.