The NDSS offers the below information on their site but I have posted it below to eliminate one extra step for you. I promise that reading this small piece of information will be well worth your time and I encourage you to also visit the NDSS site as well.
The proper use of language for “Down syndrome”:
• Down vs. Down’s - NDSS uses the preferred spelling, Down syndrome, rather than Down’s syndrome. While Down syndrome is listed in many dictionaries with both popular spellings (with or without an apostrophe s), the preferred usage in the United States is Down syndrome. This is because an “apostrophe s” connotes ownership or possession. Down syndrome is named for the English physician John Langdon Down, who characterized the condition, but did not have it. The AP Stylebook recommends using “Down syndrome,” as well.
• People with Down syndrome should always be referred to as people first. Instead of “a Down syndrome child,” it should be “a child with Down syndrome.” Also avoid “Down’s child” and describing the condition as “Down’s,” as in, “He has Down’s.”
• Down syndrome is a condition or a syndrome, not a disease.
• People “have” Down syndrome, they do not “suffer from” it and are not “afflicted by” it.
• While it is unfortunately clinically acceptable to say “mental retardation,” you should use the more socially acceptable “intellectual disability”. NDSS strongly condemns the use of the word "retarded" in any derogatory context. Using this word is hurtful and suggests that people with disabilities are not competent.