The chances increase to 1 in 400 by age 35, and 1 in 100 at age 40. A previous child or pregnancy with a birth defect. A woman may be offered CVS or amniocentesis during subsequent pregnancies if she already has had a child or pregnancy diagnosed with a chromosomal abnormality or a wide range of genetic birth defects. Other family history. Couples without a previously affected child also may be offered prenatal testing if their family medical histories indicate their children may be at increased risk of inheriting a genetic disorder.
Prenatal testing is offered only if the suspected condition can be diagnosed before birth.
Prenatal diagnosis is possible for virtually all chromosomal disorders, but not all genetic ones. CVS usually is not recommended if a woman has bleeding or spotting during the pregnancy. CVS requires taking a small piece of the chorionic villi, which are wisps of tissue that attach the pregnancy sac to the wall of the uterus. A laboratory analyzes cells from the villi, which normally have the same genetic and biochemical makeup as the fetus. Test results are generally available in about 10 days, although preliminary results may be available sooner.