Too much or too little genetic information (chromosome material) can cause abnormal development of the fetus or death. Each year approximately 2.5 million pregnant women are screened for Down Syndrome using invasive screening methods (amniocentesis or chorionic villus sampling). This 11 center study of 38,000 women will compare the accuracy of the several non-invasive tests in the first and second trimesters of pregnancy versus amniocentesis or diagnosis at birth to diagnose aneuploidy or Down Syndrome.