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Backdrop: A horror-themed campaign that heavily draws upon the Lovecraft mythos, using the Dungeons & Dragons 5th Edition ruleset.
Situation: The party has encountered a story arc inspired by the American folktale of the Jersey Devil. An NPC is currently pregnant with an aberration; the party has used various magical abilities to successfully divine the gestating creature's true nature. Thanks to previous incidents being recorded in the town's charter, the party knows that giving birth to this creature will be fatal to the NPC and the creature itself will be dramatically more difficult to deal with.
The party has access to high level arcane magic (but not the Wish spell) and wants to find a way to safely separate the NPC from the aberration growing within her. Even in its current state the aberration could pose a threat if disturbed via surgery. The idea has been posed to try using a spell that targets only the NPC, specifically the Banishment spell - sending the mother to a harmless demiplane while the aberration can be collected and dealt with, without risking harm to the NPC.
With the above details in mind, could the Banishment spell (or something similar) be used to separate the NPC from the aberration, such as by banishing just one or the other, while adhering more or less to Rules as Written?
"What happens to swallowed creatures when banishment is cast on the creature that swallowed them?" covers a similar topic, but the circumstances are different in that this is not a creature that has been swallowed; the creature is actively growing inside the NPC.