So I checked, and you don’t seem to have covered this in your What-ifs in your Game of Thrones chapter analyses, but there is one scene in Catelyn’s earliest chapters (if not the very first one), in which Catelyn is reflecting after having sex with her husband, that she was still young enough to have another child. I’m curious of the possible ramifications if she had in fact gotten pregnant, provided there’s enough time between GoT and CoK for her to give birth, and how they might differ based

(Catelyn with child what-if, cont.) (2) on what gender the child was. I could easily see the Freys choosing not to kill him/her, and instead raise it to be a pro-Frey Stark. On the other hand, Tywin might have demanded the baby handed over to be raised as a ward of the Lannisters.

If Catelyn got pregnant in Catelyn II of AGOT, she would have given birth roughly around Catelyn I of ACOK. This has some pretty wild ramifications:

• first and most obviously, she’s not going to Bitterbridge and Storm’s End having just given birth – Robb’s going to send someone else. This has substantial knock-on effects for Brienne of Tarth, which in turn reduces the odds that Jaime will be released.
• Second, if her child is a boy, the Red Wedding loses a good deal of its political purpose, because the Stark/Tully forces will have a boy king to rally around. 
• Third, no way in hell Blackfish lets his niece and his king become Lannister prisoners.