Not improbable at all. My mom had me at age 39 and my younger brother four years later.
Think of all the difficulties that could come with raising a child at that age that might convince you it was a good idea.
WHAT "difficulties"?? To be forty-two is not precisely to be on ones's deathbed. One might argue that someone of that level of maturity would have fewer "difficulties" than someone fourteen.
Well, when you come right down to it, there's no such thing as a difficulty that justifies or mitigates killing a child. But you have to admit that a lot of women that age would think they were supposed to be done raising children. Often they're looking at the next phase of their life, and they may convince themselves that they are likely to have health problems.
My wife is 41, and if she got pregnant again we wouldn't even worry about it, but not everyone has that point of view, and they take the easy way out.