During Episode 5 of the latest season of GoT, fans saw a conversation between Cersei and Jaime where she revealed to her brother/lover that she was pregnant. However, because of Cersei's almost official status as the main villain of GoT, naturally some fans are questioning whether or not she's been telling the truth. It's possible that Cersei's not actually pregnant, which wouldn't be all that surprising considering the character's history of not always being totally honest. And now, it's seeming more and more likely that if she really is pregnant, Cersei's baby might not be Jaime's.
For those of you wondering why Cersei would lie to Jaime about this, let's review the facts. First of all, the two Lannister siblings haven't always been on the same page, especially during the newest season where they have disagreed about how to handle situations with Daenerys, Jon Snow, and the White Walkers. While Cersei has been steadfast about defending her claim to the throne, Jaime has questioned Cersei's motives and made mention of the fact that they didn't have an heir.
So when Cersei told Jaime that she was pregnant right after he urged her to make a temporary alliance with Daenerys, it immediately seemed like she was hoping to use the information to manipulate him. Additionally, in the Season 7 finale, Cersei again attempted to use her pregnancy to convince Jaime to follow her wishes in regards to Dany and the White Walkers.
GoT fans on Twitter have floated a handful of possibilities of who the father of Cersei's baby could be, if not Jaime. Some have suggested it could be The Mountain's, which is possible, but the show hasn't really hinted at any romance between Cersei and The Mountain. Or perhaps it could be Euron Greyjoy's baby? Cersei did promise to marry him, and we learned that the two were still working together to protect King's Landing.
Whether or not she is really pregnant, and regardless who the father might be, one thing is clear: Cersei is trying to use said pregnancy to control Jaime. Unfortunately for her, we saw Jaime openly disobey Cersei at the end of Season 7, when he left King's Landing to fight the White Walkers. So she definitely seems to be losing the control she once had.
If Cersei is lying about being pregnant then it will certainly become obvious to the people around her in the near future when she doesn't start showing physical signs of being with child. And if she does have a baby, and the father is actually Jaime, then it could give the duo a proper heir that might give their soldiers a new (albeit young) leader to fight for during the ongoing war.