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MB3 173a: Preparations for a Bris Milah on Shabbos (331:5-6)
Milah is not performed on Shabbos for an אנדרוגינוס, for a child born during בין השמשות (either Erev or Motzei Shabbos), or for a child born via c-section.
Any preparations for a bris that can be done before Shabbos can't be done on Shabbos, even if it means postponing the bris.