Problem is most (if not all, not sure?) births are locked behind Erevi. Pretty sure she is the canonical "first" birth in the game.
I
think you can get Maghda first of any girl. Knock her up early (in theory on Day 1) and she can have Olga before Erevi gives birth. Have not tried it.
In theory the vampires and Rosy as well (and Vixenatrix laying her eggs!), though those require REALLY long delays before seeing Erevi again... Haven't tried those, either.
"The Goblin King" used to require completing "Waystones II" but not any more. Dunno if Erevi giving birth first is required to get SD after Goblin King is done, I think I remember a playthrough where it wasn't but that was a while back.
But otherwise, yeah, Erevi needs to give birth before anyone else can, and before many can even get pregnant.