Down syndrome occurs in approximately  1 of every 700 births in the United States; about 6000 babies with Down syndrome are born every year.  As women get older, the chance of having a baby with Down syndrome gradually increases. The chance that a 35-year-old woman will have a baby with Down syndrome is about 1 in 350. The chance increases to 1 in 100 for a 40-year-old woman and 1 in 30 for a 45-year-old woman. Although older mothers have an increased chance of having a baby with Down syndrome, most babies with the condition are born to mothers under 35 simply because they have more babies.

Down syndrome is caused by having three copies of chromosome 21 instead of the usual two copies; that is why the condition is also known as trisomy 21. There are three types of Down syndrome. The most common type is due to nondisjunction meaning that one parent passes on two copies of chromosome 21 to the baby instead of one. This leads to the baby inheriting a total of three copies of chromosome 21. Down syndrome due to nondisjunction (95% of cases) occurs by chance. The chance of recurrence for trisomy 21 or other trisomy is 1% or the maternal age-related chance, whichever is higher. 

There are two other types of Down syndrome.  Mosaic Down syndrome means that the baby has some cells with trisomy 21 and some cells that have the usual two copies of chromosome 21. The chance of recurrence is similar to that for nondisjunction trisomy 21 (up to 1% or age-related risk). The third type of Down syndrome is the translocation type. In this case, the baby’s extra copy of chromosome 21 is attached to another chromosome. In about 2/3^rds of cases, the translocation happens only in the baby. However, in about 1/3^rd of cases, one of the parents has a balanced translocation involving chromosome 21. The parent only has two copies of chromosome 21, hence balanced, but one is translocated to a different chromosome. This increases the chance of having another baby with Down syndrome. The precise chance depends on whether the parent carrying a translocation is male or female and which chromosomes are involved in the translocation.