Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (Trisomy 21) or in part (such as due to translocations). The effects of the extra copy vary greatly among people, depending on the extent of the extra copy, genetic history and pure chance. In 2007, the American College of Obstetricians and Gynecologists (ACOG) endorsed guidelines that offer risk assessment to all pregnancies for fetal chromosomal abnormalities, including Down syndrome. It is estimated that approximately 70%, or 2.8 million, women undergo Down syndrome screening in the United States each year.