Families have often been affected by these conditions without realising it because up to half of all pregnancies are miscarried, maybe before a woman realises she is pregnant, and one in five 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.

Maternal Age

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 that tend to occur more often when a mother is older. Only rarely is there a previous history of such a birth.