There is no literature on this topic, so you will be left with the preference of your treating MD. In general, I do not alter OB practice in my pregnant Chiari patients. In my opinion, unless the literature is clear, don't change things(and there is no clear literature in your situation).
As Beeba said, discuss the situation with your OB and see whomever they recommend. Getting everything in order beforehand certainly makes sense(meaning making sure OB and anesthesia are on your page, which may require neurosurgical clearance).