Differences are not measured in absolutes but are relative. As an example that applies to this case, the difference between 14 and 15 weeks is an infinite order of magnitude greater than the difference between 28 and 36 weeks, that latter of which is much larger in absolute terms. Why? A normally developing child with a BD mother has an excellent chance of survival regardless of whether the child is 28 weeks or 36 weeks. However, in the case of the former. We are dealing with an extraordinary low chance; one human being in the entire history of mankind in one case, versus no such case in the other. It is more probable that the mother will rise from the dead, the mother will be assumed into heaven, or that the Red Sea will part.