At what stage of pregnancy?

Amniocentesis can be carried out from the 13th week of pregnancy, and the safest time to have it is after the 15th week. It is always offered to women over 38, but also to women who fall into at-risk categories.

Your blood test results are anaylsed for the presence of certain hormones (alpha-foeto-proteins, chorionic gonadotrophic and estriol hormones) which indicates an increased risk of Down's. A scan is also performed.

Amniocentesis can also be performed at a later stage of your pregnancy, in the third trimester, if your pregnancy is thought to be a risk; if you have incompatible blood groups, for example, or if the doctor suspects the baby could have digestive or neurological malformations or need medical attention.