Most people with Down syndrome have 47 chromosomes, rather than 46. They have an extra copy of chromosome 21, the smallest human chromosome. This is responsible for the features of Down syndrome, including delayed development, some learning difficulties and possible heart problems. The chances of a couple having a baby with Down syndrome depend on the mother's age. For women under the age of 25, the chances are about one in 1400, rising to one in a 100 by age 40.