In each cell of a baby with trisomy there is one set of three identical chromosomes among the normal pairs.

Trisomy 18 Karyotype

This photograph  shows three number 18 chromosomes among the normal pairs. Trisomy 13 was named Patau's syndrome after the doctor who in 1960 identified the extra number 13 chromosome, and trisomy 18 was named Edwards' syndrome after Dr John Edwards who identified the extra number 18 chromosome.  

What happens

The extra chromosome may be in the egg cell produced by the mother or the sperm cell produced by the father. It has been estimated that as many as 95 out of every 100 embryos with trisomy 13 or 18 are lost in early pregnancy. The extra genetic material affects every stage of development of an affected baby, and these trisomies are the most common chromosomal disorders after trisomy 21 (Down's syndrome). Trisomy 13 affects equal numbers of boys and girls whereas babies with trisomy 18 are three times more likely to be girls.