Hey, I have a theory that Historia was pregnant because of Eren, not Historia's childhood friend. Why? I remembered a page when Eren monitoring those two (that I believe, Historia and Eren want the unknown guy to acknowledge the baby was "created" by this unknown guy to manipulate government and keep it very secret only for them) IT'S F-ING IMPOSSIBLE that Eren has high confidence to meet Zeke (and his declaration of war to Marley) without plan B.
Yep, his plan B is Historia.