This is the later years of Eiria and Fenth!!
So to ease frustration:
After Eiria had completed her surgery she was told she can no longer carry children. And being the fact that she can fuck Fenth, she took her chance when all their existing kids moved out of the house.
So here’s Fenth, all shocked and confused. As literally 20 minutes later, he goes into labor and has Benjamin.
I hope this explains it all.