Apparently succubus babies only take half a year to come to term. Also iirc children of a succubus are either 100% the fathers race or 100% succubus, with the latter being less common(?).
If I had to wager based on what Ive seen.
Spoiler:
Mercie probably had an arranged marrage of some sort coming up when she fled to the human realm
meets up with Naoya, gets preggers, has to leave Asuha with Naoya for political reasons