Why Trisomy 13 or 18?

Babies have been born with trisomy 13 and trisomy 18 since the beginning of time, but it was only in 1960 that Dr. Patau and Dr. Edwards identified the respective features of these syndromes. Both syndromes are the result of an extra l3th or l8th chromosome, respectively, being present in each cell.

We know what happens, but in the majority of cases nobody knows why a baby is conceived with trisomy 13 or 18. Parents should not feel that anything they did or failed to do was responsible for their baby’s chromosomal disorder.

One in four of all pregnancies ends in miscarriage and a high proportion are thought to be caused by a chromosomal disorder. A high proportion of trisomy 13 and trisomy 18 pregnancies end in miscarriage within twelve weeks of conception. It is therefore quite rare for such a pregnancy to continue to full term. The incidence of trisomy 13 is about 1 in 5,000 of total births while that of trisomy 18 is about 1 in 2,500 of total births. As the female X chromosome is more viable, there are more girls born than boys.

The risk of conceiving a baby with a chromosomal disorder increases with age. The average age of a mother giving birth to a baby with trisomy 13 or trisomy 18 is 32 years. However, as most babies are born to mothers in their twenties and early thirties, many babies with trisomies 13 and 18 are born to women of this age.

Unless there is a genetic problem, it is extremely rare to have a second baby with the same disorder. However, if you are considering having another baby it is worthwhile to seek the advice of a genetic counsellor.

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