Down's syndrome is one of the most common birth defects, affecting around one in every 800 babies born. It most often occurs when the nucleus of cells contains three chromosome-21's, rather than the usual two. Women who are over age 35 or who already have a child with the disease are at highest risk of having a child with Down's syndrome. It can be detected during pregnancy with a procedure known as an 'amniocentisis' (am-nee-oh-sen-TEE-sis), usually done between the fourteenth and twentieth weeks. A small amount of amniotic (am-nee-AH-tic) fluid is withdrawn from the sac that surrounds the fetus (FEE-tus) through a long needle inserted through the mother's abdomen. The fluid is then studied under a microscope. Down syndrome results in a child having a number of physical abnormalities, as well as mental retardation. Early detection is very important, because it also may be accompanied by other abnormalities of the heart, intestinal tract, thyroid, or blood. There's no cure, but there are specially-designed therapies to help a child with Down syndrome make the most of his developmental and physical capabilities.