The expert choose to withhold information from the mother leaving her with the subjective probability of 25% of having a sick baby. The question now becomes this: what was the true level of knowledge of the expert (as opposed to the mother)? Lets suppose that the probability of the test showing a mutation in the right position when this position is in fact wild-type is 5% (a fairly sloppy research scientist might be assumed to mislabel or otherwise mix up 1 test tube in 20, sequencing error-rate must be much lower). Lets further suppose that the probability of the test outcome being “wild-type” when the mother is in fact a mutation carrier is also 5%.

Using the Bayes rule we can calculate that the expert should now believe with 95% certainty that the mother is a carrier.
If the expert believed that only 1% error-rate applies, then the level of his belief should rise to 99%.

The mother may know very little about genetics but this is surely not a valid reason to deprive her of the chance of interpreting the statement: „it is over 95% probable that your baby boy will have a fifty-fyfty chance of having the disease“? Or put differently: would it be a good idea to wait several months in order to be able instead to say: „it is over 99,9% probable that your baby boy will have a 50:50 chance of being sick“? It seems doubtful to me that the difference between 95% and 99,9% would be enough to change the mother‘s behaviour.