A son of Robb would not add much to the story - the ASOIAF story is going to end while such a child would still be too small to be a proper character. It would needlessly complicate the Winterfell succession plotline.
The only way GRRM could make a pregnant Jeyne plotline meaningful would be if Littlefinger or Roose Bolton found out about the switch and tried to hunt her down. Ultimately though the plot line would have to end with the miscarriage or death of the baby, so we can get back to focussing on the more important Winterfell characters.
Thats what I was thinking.
Even if Jeynde did escape Riverrun pregnant with Robbs' child, it won't do much rather then have people hunt her down and kill her. Otherwise, by the child is actually of age to do significant things, the story will probably be over.
As much as I like Robb, I'd rather see Bran, Sansa, Arya, Rickon, or Jon reclaim Winterfell rather then his child.