Personal fan theory to Finis pregnancy in finale. Baby fathered by "original" Leobort NOT the Leobort in finale. In Finis memories they show candlelight silhouettes of them touching intimately which I think alludes to love making. After original Leo dies and Finis sings the song of mortality and gains immortality, changes to her body is suspended. She can't age, she can't be damaged (based on her attempted drowning, being burned at the stake and being pushed off a bridge).
After Rin's sacrifice, hope (her song of healing) is returned to Finis. There is a brief flash where we hear a heartbeat and Rin's shock. I think this was to signify, she was released from her immortality and that perhaps she had become pregnant by Original Leo but the immortality placed it in a suspended state. The shock and heartbeat was NOT Finis's heart but the heartbeat of the child which surprised her. What do you think?

Was awesome till they fucked it up with a time loop storyline.