I wonder about this. Most people at the banquet were killed by having their throats slit, it seemed like Robb's wife was deliberately stabbed repeatedly in the womb, as if it was to kill the baby. And earlier during the episode, when Walder Frey was looking at her, he said something like "I know exactly what is going on beneath a dress" or something along those lines, he was being crude alongside that so we obviously assumed he meant 'you have a nice bum lol', but seeing how everything panned out, I wonder whether he'd guessed she was pregnant and instructed his man to kill her that way.
Yeah, I gave the episode a 2nd viewing and that was made very clear. At the time you assumed he was being a rude cnut and talking about her arse/snatch. But based on the stabbing, it was obvious he was implying he knew about the pregnancy.
I also noticed the young bride looked at Rob during the wedding. Again, at the time you thought she was staring at him thinking "what may have been". But now you wonder if she was staring because she knew what was going to happen. Seemed a weird shot to me.