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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 or the 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 cause dby 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.


Trisomy originates in the egg cell or sperm cell and results in 47 chromosomes instead of 46.

Why does it happen?

Nobody knows. Every pregnant woman has a slight risk of having a baby with a chromosome defect which affects about one in every 200 births.The risk of having a baby with trisomy 13,18 or 21 (Down’s Syndrome) increases with age, but most babies with trisomy are born to mothers in their twenties because more babies are born to women in this age group. Except in rare cases that are inherited, chromosome defects are usually one-off events which tend to occur more often when a mother is older. Only rarely is there a previous history of such a birth.

"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.

Dr. Ian Young: Professor of Paediatric Genetics former SOFT Medical Advisor

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