What is Down syndrome?
Down syndrome (Trisomy 21) is the most common cause of mental retardation. Affected children have varying degrees of physical and mental handicap, and often have birth defects involving the heart or the digestive tract. Down syndrome affects approximately 1 in 700-800 pregnancies, and occurs when the fetus has an extra copy of chromosome 21. Although Down syndrome is more common when the mother is 35 or older, women of any age can have a fetus with this condition.
What is Trisomy 18?
Trisomy 18, or Edwards syndrome, is more severe than Down syndrome. The majority of fetuses with Trisomy 18 are stillborn. Those that are born alive have severe mental retardation, multiple birth defects, and usually die in the first 2-3 weeks of life. Trisomy 18 occurs when the fetus inherits an extra copy of chromosome 18. Like Down syndrome, Trisomy 18 occurs more commonly if the mother is 35 or older, but women of any age can have a fetus with this condition.
What if I will be 35 or older at the time of my due date?
Women of any age can have a child with Down syndrome or Trisomy 18. However, the risk increases as a woman gets older. At age 35, the risk of having a child with Down syndrome and the risk of losing a pregnancy as the result of having an amniocentesis are about equal — 1 in 350. So at age 35 or older, a woman may feel that her age-related risk of Down syndrome is high enough to justify a diagnostic test (e.g., amniocentesis or CVS) without having a screening test first. Other women this age may prefer to have a screening test and then decide about amniocentesis or CVS. The ultimate decision is up to you.
What if I had a previous child with Down syndrome or Trisomy 18?
As long as your previous child had the classic form of these disorders due to trisomy, not translocation, then your recurrence risk is 1% or your age-related risk, whichever is higher. A prenatal diagnostic procedure is offered to most women with this history, but the final decision is up to you.