The tri in the word trisomy means three. In each cell there is one set of three identical chromosomes among the normal pairs.
Trisomy in pregnancy
The extra chromosome may be in the egg cell produced by the mother orthe sperm cell produced by the father, and the additional genetic material affects every stage of the development of the baby, rather like too much information on computer software would prevent a computer from functioning properly. It has been estimated that as many as 95 out of every 100 embryos with trisomy 13 or 18 are lost in early pregnancy.
It is not generally realised that up to half of all pregnancies are miscarried, often before a woman realises she is pregnant, and 1 in 5 confirmed pregnancies end naturally within 3 months. Many of these losses are caused by a chromosome abnormality, but a woman is unlikely to be offered chromosome tests to confirm the reason unless previous problems during pregnancy have indicated this might be a cause.
Why does it happen?
"In many instances we don’t know the answers. Why it happens is a mystery.We know what happens but nobody knows why it happens.”
Ian Young Professor of Paediatric Genetics
Does the age of the mother affect the risk?
About 1 in 200 births result in a baby with a chromosome defect. The risk of trisomy increases with age, but most babies with these conditions are born to mothers in their early twenties because more babies are born to women in this age group. Except in rare inherited cases, chromosome defects are usually one-off events.
Trisomy 13 (Patau's syndrome) and Trisomy 18 (Edwards' Syndrome)
Trisomy 13 and trisomy 18 births occur about once in every 1,800 pregnancies and are the most common autosomal trisomies after trisomy 21, which is also known as Down syndrome. Trisomy 13 affects equal numbers of boys and girls whereas babies with trisomy 18 are three times more likely to be girls. In England and Wales between 2004 and 2011, there were 4,028 diagnosed cases of trisomy 18 and 1,662 of trisomy 13. More than 6 in every 10 pregnancies with either condition were terminated (64% for Edwards's syndrome and 63% for Patau's syndrome). In the same 8 year period, 372 babies with trisomy 18 were born alive as were 173 with trisomy 13. (National Down Syndrome Cytogenetic Register Annual Reports 2008/09 - data are provisional).