Lisa Is 35 and Pregnant for the First Time

Multiple Choice

Lisa is 35 and pregnant for the first time. As a "mature" mother, she is worried about the possibility of having a baby with Down Syndrome. Chances of a 35-year old mother having a baby with Down Syndrome is 1 in 300. Just to be on the safe side, Lisa 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 90% of the time. There is, however, a small "false positive" rate of 7%. Lisa's blood serum test provides a positive result for Down Syndrome. The posterior probability that Lisa's child will have Down Syndrome is:

A) 0.037 or 3.7%.
B) 0.003 or 0.3%.
C) 0.03 or 3%.
D) 0.9 or 90%.