I definitely think that some women, based on experiences, can hold on to a pregnancy longer than they might have originally.
However, I also feel like sometimes women with extreme trauma/pain/grief can just as easily birth very quickly and smoothly.
I've seen people not birth until someone important arrives or leaves.
I'm not too sure about fetal development slowing at all, though. I do know that there is a sharp rise in stress hormones found in babies whose mothers experience pain, abuse, trauma, or grief. Perhaps this could, in fact, create some slowing of development. However, it is my personal belief that the onset of labor is more than just baby being ripe - so it could be *one* factor of many, I suppose.