What is Down syndrome, and what causes it?
Down syndrome is a genetic disorder. Most people have 46 chromosomes in each cell. Children with Down syndrome have 47 chromosomes. They have an extra copy of chromosome 21. Normally, we inherit 23 chromosomes from our mother and 23 chromosomes from our father (for a total of 46). Babies with Down syndrome inherit an extra copy of chromosome 21 leading to 3 copies (one from Mom, one from Dad, plus one extra). We call this Trisomy 21. Some children have mosaic Down syndrome or mosaicism. In this type of Down syndrome, not all cells have the extra chromosome, which can result in the child being less severely affected. To learn more about genetics and to better understand how genes cause syndromes, see YourChild: Genetic Syndromes. How common is Down syndrome? Down syndrome is the most common single cause of human birth defects. About 1 out of every 660 babies is born with Down syndrome.[1] How is it diagnosed? Down syndrome can either be diagnosed in utero (via amniocentesis) or, most co
