CDC and Emory University scientists examined maternal smoking and oral contraceptive use as possible risk factors for Down syndrome of maternal origin. This is the first epidemiologic study to categorize Down syndrome by parental origin and timing of the chromosome error before assessing possible risk factors. 

• Down syndrome, the most commonly identified cause of mental retardation, occurs in about 1 in 800 births. Despite many years of research to identify risk factors associated with Down syndrome, only one factor, advanced maternal age, has been well established.
  

• Previous studies of risk factors for Down syndrome have pooled all cases regardless of parental origin or timing of the chromosome error. With new DNA technology and chromosome 21-specific genetic markers, determining the parental origin of the chromosome error and the timing of that error during meiosis is possible. [Meiosis is the special process of cell division that creates egg and sperm cells so that each has half the number of chromosomes normally found in other cells in the body.]
  

• Younger mothers (<35 years) who smoke and have meiotic II error are at an increased risk of having children with Down syndrome. The combined use of cigarettes and oral contraceptives increased the risk even further.
  

• This is the first population-based epidemiologic study to categorize Down syndrome cases by the parental origin and the timing of the chromosome error to identify important environmental and maternal health-related risk factors.
  

• Our results show that categorizing cases of Down syndrome by parental origin and timing of the meiotic error allows for more precision in identifying risk factors and may shed light on mechanisms of meiotic error.

Results published in: Genetics in Medicine 1999;1:80-88.

Date: October 5, 2005
Content source: National Center on Birth Defects and Developmental Disabilities