Downs syndrome is the most common, serious, chromosomal disorder, occurring in 1 out of 800 live births. Most often an affected child has three copies of chromosome 21 (ie trisomy 21). Older mothers are at dramatically increased risk for having a child with Downs syndrome. At 35 years of age, the risk of birth is 1 in 385, at age 40 it is 1 in 105. All mothers who will be 35 yr or older at the time of birth should ideally be offered amniocentesis for fetal karyotype determination. This strategy will detect about 20% of all fetuses with Downs syndrome.
Approximately 1 in every 50 mothers classified with abnormal results of Triple Test will be found to have a fetus of Down syndrome.