One thought on Cersei's pregnancy is that it could perhaps be Euron's.
I suppose it's possible sleeping with Cersei was one of his conditions for providing his fleet, and the sex scene between her and Jaime last episode could be her coving it up, and misdirection from the showrunners. We know from Maggie the Frog's prophesy that the child wont live, so Jaimie killing her over this (also fulfilling the prophesy) might be a plausible outcome. After all, if Jaimie really is Cersei's killer, then currently in the show there isn't any justifiable reason for him to do it.
Or the child really is Jaimie's, and is just a device to explain why Jaimie is willing to stick by her.