Let's set pregnancy aside for a moment and consider some other aspects of this issue.
If you have harmful microbes in your body, does this spell leave them behind? If so, then it's effectively a Cure Disease for anything caused by foreign pathogens. That's the good news, but...
Every human body contains a host of symbiotic bacteria that help us with digestion and fighting off pathogens. Does banishment leave these behind? If so, anybody affected by this spell is likely to get very sick if not die from opportunistic infections and inability to digest.
Some RL creatures use symbiosis for other purposes. Green tree sloths have symbiotic algae living in their fur; the algae helps camouflage them and provides nutrition. If you banish a sloth, does it lose its green camouflage and suffer health effects from loss of nutrition?
Aside from the obvious issues of taste and player discomfort, applying a strict interpretation of "one creature" risks bogging your game down in endless argument about this sort of issue. (What exactly are the effects of losing your symbiotes? Do elves and dwarves have the same kind of symbiotes as humans?)
Now consider multi-headed fantasy monsters like ettins, hydras, and chimeras. In some cases their heads are capable of independent thought, even arguing with one another. But for D&D's purposes, a multi-headed monster is still considered a single creature. Otherwise, banishment would presumably kill all but one head. Given that a fetus has less autonomy than an ettin's second head, it seems consistent with existing practice to treat it as part of the parent.
(Note that teleportation effects generally take a generous interpretation of what counts as a single person - usually you get to keep your clothes and equipment.)
The mention of "a harmless demiplane" would seem to suggest that this spell was not intended to cause other kinds of harm - else presumably the spell description would mention it.