Question
At 18 weeks of pregnancy, a Down syndrome screening test shows a risk coefficient of 1 in 1020, an 18-trisomy risk of 1 in 400, a 21-trisomy risk of 1 in 3500, and an A/P risk of 1 in 490. Are these risk coefficients high or low? What does the 18-trisomy ratio indicate? Is there a high possibility that the fetus will have Down syndrome?
Answer
Down syndrome is caused by chromosomal abnormalities, primarily due to errors in the division of the twenty-first pair of chromosomes during genetic recombination, or due to translocation. Generally, between weeks 15 to 20 of pregnancy, a chromosomal test via venous blood can determine whether the condition is present. If a high-risk diagnosis is made, amniocentesis can be performed to extract amniotic fluid for a more accurate test, but it carries some risk.