My comment was written from what I know will happen from spoilers from the og novel. Sovieshu is the infertile one. Rashta's baby daddy is the same dude as her first kid's.
Navier will get pregnant (will happen a little after Rashta's baby's birth). Sovieshu finds out and goes insane for several reasons: he's the infertile one, the child he had been eagerly expecting isn't his, and he gave up his life with Navier for nothing.
He's eventually going to meet her children at the end of the novel and he gets even more sadder because the twin girl looks just like Navier; and it was a dream of his having a baby girl who looked like Navier.
He suspected both of them were infertile, because they ate poisoned cookies when they were young.
When rashta was pregnant, he assumed it was it was just the empress.
But she gets pregnant later in the story.
I'm guessing, the poison had an affect on both of them, making them incompatible. Since that's a real thing that can happen.
But since they each found someone that didn't have fertility issues, they were able to conceive a children.
...PREGNANT???? I CAN'T WAIT ANYMORE IDC ABOUT TRASHTA AND HER WANNABE MACHINATIONS, I JUST WANT TO FINALLY SEE THE SCENE, DRAWN, OF WHEN SOVIESHU'S AID TELLS HIM THAT NAVIER IS PREGGO AND HE STARTS GOING CRAZY I already know that Heinry and Navier are a power couple, just give me the drama already ffs