Down syndrome is caused by an abnormality in chromosome 21. However, the specific mechanism that causes the abnormality is not known. Some factors have been identified that may increase the risk of having a child with Down syndrome. These risk factors include:

• Advanced parental age. The mother's age is the most prominent and best-established risk factor. The older a woman is when she gives birth, the greater her chance of having a child with Down syndrome. Children born to women older than 35 have a much higher risk of the condition. A 30-year-old woman has a one-in-1,000 chance of having a child with Down syndrome, according to the American Academy of Family Physicians (AAFP). However, this risk increases to one in 400 at age 35 and one in 60 at age 42.
  
  In addition, research in recent years has increasingly indicated that advanced paternal age also increases the risk of several health problems in children, including birth defects such as Down syndrome. Sperm declines in quality as men age, swimming more slowly and becoming more genetically defective. One recent study found that men's sperm usually started showing signs of decline at age 56.
  
  
• Parental chromosomal abnormality. Men and women with certain chromosomal abnormalities have an increased risk of having a child with Down syndrome. In some cases, the parent does not have any extra or missing genetic material, but genes are not located in the normal places. For example, a part of chromosome 21 may occur on another chromosome but be missing from chromosome 21.
  
  
• Previous child with Down syndrome. If parents have had one child with Down syndrome, they have a slightly increased chance of having another. This may be due to a chromosomal abnormality that has not yet been detected.