Answer:
3.74% probability that her baby has T18
Explanation:
Bayes Theorem:
Two events, A and B.

In which P(B|A) is the probability of B happening when A has happened and P(A|B) is the probability of A happening when B has happened.
In this question:
Event A: Positive test.
Event B: The baby having T18.
T18 occurs in only 1 in 2500 pregnancies in the U.S.
This means that

The probability of a positive test result for a baby with T18 is 0.97.
This means that

The overall probability of a positive test result is 0.010384.
This means that

What is the probability that her baby has T18

3.74% probability that her baby has T18