A trocar for placement in the lumen of a cannula to facilitate inserting the cannula through the wall of a body cavity, the trocar comprising an obturator with a point for piercing the wall of the body cavity, and a protective sleeve mounted concentrically around the obturator. The obturator retracts proximally relative to the protective sleeve after the trocar point has penetrated through the wall of the body cavity. The protective sleeve is mounted for axial movement relative to the obturator, retracting proximally in response to drag from the wall of the body cavity as the trocar is advanced through the wall, and advancing distally after the cannula has penetrated through the wall, reducing the drag on the sleeve. The distal advancement of the sleeve triggers the retraction of the obturator.