Testing pregnant women's blood for disorders in unborn children promises dramatic advances in medicine, researchers have said.A US team writing in the New England Journal of Medicine say Down's syndrome can be reliably tested for in the mother's blood. Meanwhile, Great Ormond Street Hospital has started offering similar tests.A decision on whether the UK's Down's syndrome screening programme should change is due this year.At the moment in the UK, a woman is assessed based on her age and an ultrasound scan, with those deemed high-risk having further tests.These involve a needle taking a sample of the placenta or the fluid that bathes the baby. There is a risk of miscarriage with the procedure.Blood tests look for fragments of DNA from the placenta, which drift about in the mother's bloodstream.Down's syndrome is caused by an extra copy of a huge stretch of DNA and that extra bundle of genetic information can be detected in the blood.If initial tests are ...