The spell specifies the following:
You attempt to send one creature that you can see within range to
another plane of existence. The target must succeed on a Charisma
saving throw or be banished. If the target is native to the plane of
existence you’re on, you banish the target to a harmless demiplane.
While there, the target is incapacitated
Does the spell take effect and if so does it bring the as yet unborn creature(s) with the mother? Possible complications include, beyond the spell only targeting a single creature, that it must also be a creature that you can see. The supposition is that being inside another creature would provide those inside with Total Cover.
A target with total cover can't be targeted directly by an attack or a
spell, although some spells can reach such a target by including it in
an area of effect. A target has total cover if it is completely
concealed by an obstacle