Linda Is 22 and Pregnant for the First Time

Multiple Choice

Linda is 22 and pregnant for the first time. Chances of a 22-year old mother having a baby with Down Syndrome is 1 in 1000. Just to be on the safe side, Linda decides to have a maternal blood serum test to check for chromosomal abnormalities. If the baby has Down Syndrome, then the blood serum test will deliver a positive result 86% of the time. There is, however, a small "false positive" rate of 5%. Linda's blood serum test provides a positive result for Down Syndrome. The probability that Linda's child will have Down Syndrome is (approximately) :

A) 86%.
B) 1.7%.
C) 0.1%.
D) 63%.