As yet we do not know what causes the presence of an extra chromosome 21. It can come from either the mother or the father. There is no way of predicting whether a person is more or less likely to make and egg or sperm with 24 chromosomes. There is a definite link with advanced maternal age for reasons yet unknown. However most babies with Down’s syndrome are born to women under the age of 35, as younger women have higher fertility rates. What we do know is that no one is to blame. Nothing done before or during pregnancy can cause Down’s syndrome. It occurs in all races, social classes and in all countries throughout the world. It can happen to anyone.