It's always best to show up for interview and ceremony VS rescheduling it. Why? Because sometimes USCIS may take a while to reschedule it. Some cases get in the "black hole", taking months and sometimes years for somebody to pay attention to them at USCIS.
However, giving birth is very good reason for rescheduling interview or oath. If she can't attend, she can't attend.
Many cases get approved within 6 months. So maybe she'd be lucky and naturalize before December.