TRISOMY
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.
Trisomy originates in the egg cell or sperm cell and results in 47 chromosomes instead of 46.
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 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 the 12 months April 2008 to April 2009 there were 172 diagnoses of Patau and 495 diagnoses of Edwards’ syndrome. 91% of Patau and 92% of Edwards syndrome diagnoses were made prenatally, and 65% of Patau and 68% of Edwards’ pregnancies were terminated. It is estimated that there were 18 live births with Patau syndrome and 37 live births with Edwards’syndrome. (National Down Syndrome Cytogenetic Register Annual Reports 2008/09 - data are provisional).
HOLOPROSENCEPHALY (HPE)
Holoprosencephaly is when the front part of the brain fails to develop into clearly separate right and left halves. Generally this severely affects the function of the brain, although among children with this condition there is considerable variability in the degree of malformation and its effects. Babies diagnosed with this condition usually have a small head, excessive fluid in the brain, cleft lip, varying degrees of learning difficulties, epilepsy, hormonal problems, or problems with the heart, bones, bowel or bladder. Although a baby with holoprosencephaly may not have abnormal chromosomes, it is common in trisomy 13.
