Down Syndrome is not a common disorder nor is it rare. About one pregnancy in about 500 pregnancies will result in a baby with Down Syndrome. Of those 500 who will test positive for having a baby during labor with Down Syndrome, there is a margin of error which means that a small percentage of these women will test positive for the disorder, but they will not produce a baby with Down Syndrome.
That rate is about 5%. About six thousand babies are born each year with Down Syndrome. The number has actually increased since 1979 to more recently in the early 2000s. Mothers who have children later in life stand a greater chance of giving birth to a child with Down Syndrome.