Down syndrome (also called trisomy 21), trisomy 18 and trisomy 13 are chromosomal conditions. Chromosomes carry our genetic information. Typically, a person has 46 chromosomes in each cell of his or her body. Any time a child receives extra or missing chromosomal material, developmental problems may occur. These defects are diagnosed by amniocentesis or CVS.
An individual with Down syndrome has 47 chromosomes in each cell of his or her body because he or she has an extra copy of chromosome 21. The severity of mental retardation varies among affected individuals (typically mild to moderate) and cannot be determined until after birth. Heart defects are also found in approximately one-half of babies with this condition.
Babies with trisomy 18 and trisomy 13 also have 47 chromosomes in each cell of their bodies instead of the typical 46. These conditions are more severe than Down syndrome. Both conditions cause profound mental retardation and multiple birth defects. Few babies with either condition live more than one year, although some do survive into childhood. Like Down syndrome, the chance of having a baby with trisomy 18 increases with maternal age.
Women who are 35 or older at the time of delivery have an increased risk of having a baby with Down syndrome or other chromosome abnormality. This group of women is often referred for genetic counseling and ultrasound and is offered optional diagnostic procedures including amniocentesis and CVS. Even though a woman is not considered to be at a high risk until she reaches 35 years of age, babies with Down syndrome can be born to women of any age. The following table provides the age-related risk for having a child with Down syndrome and the risk to have a baby with any chromosome problem, including Down syndrome.
|
Maternal Age |
Down Syndrome |
All Chromosome Abnormalities |
| 25 |
1/1,205 |
1/476 |
| 30 |
1/885 |
1/384 |
| 35 |
1/365 |
1/178 |
| 40 |
1/109 |
1/63 |
| 45 |
1/32 |
1/18 |