Non-invasive blood test can detect Down\'s syndrome

A blood test can accurately detect Down\'s syndrome and other genetic fetal abnormalities in the first trimester, British researchers say. Dr. Kypros Nicolaides of the Harris Birthright Research Centre for Fetal Medicine at King\'s College London and colleagues said the blood test -- cell-free DNA -- is the first to prospectively demonstrate the feasibility of routine screening for trisomies 21, 18, and 13. A trisomy is an abnormal number of chromosomes. Current screening for Down\'s syndrome, or trisomy 21, is done between the 11th and 13th weeks of pregnancy. It involves an ultrasound screen and a hormonal analysis of the pregnant woman\'s blood. Only chorionic villus sampling and amniocentesis can definitely detect or rule out fetal genetic abnormalities, but these are invasive to the pregnancy and carry a risk of miscarriage, Nicolaides said. Down syndrome is caused by extra genetic material from chromosome 21. Each person normally has 23 pairs of chromosomes, or 46 in all. When an egg and sperm join together, they normally form a fertilized egg with 46 chromosomes. However, sometimes something goes wrong before fertilization and a developing egg or sperm cell might divide incorrectly, sometimes causing an egg or sperm cell to have an extra chromosome number 21 and the resulting embryo has 47 chromosomes instead of 46. Down syndrome is called trisomy 21 because affected individuals have three number 21 chromosomes, instead of two. The study involved 1,005 pregnant women who were tested at 10 weeks. The study, published early online in Ultrasound in Obstetrics & Gynecology, found both the cell-free DNA test and combined testing detected all trisomies, but the estimated false-positive rates were 0.1 percent using fetal cell free DNA and 3.4 percent for the combined testing.