An embryo replacement catheter has a flexible extruded shaft of a transparent polyurethane with a bore extending along its length. Gas bubbles of a diameter in the range 5.0 mu. to 10.0 mu. are incorporated into the thickness of the wall of the shaft by adding gas during extrusion. The bubbles are selected to increase the visibility of the catheter under ultrasound imaging whilst still enabling material flowing along the catheter to be seen. The inventive catheter is not confined to embryo replacement catheters.