THE BYSTANDER APPEARED shocked. Open-mouthed, she was standing there shaking her head. “Not one”, she said, “not one but two. Don’t you know that kind of thing is preventable now?”

The objects of her disgust were the two children of Sarah D., a woman I met in the United States. Sarah’s children were both born with an extra chromosome 21, giving them the typical appearance of Down’s syndrome. Sarah was aware that Down’s syndrome can be prevented – or rather, that the condition can be diagnosed in a foetus long before birth, allowing for what is rather gruesomely termed a “therapeutic abortion”.