An obstetrical vacuum extractor has a bell-shaped cup with an open edge in communication with a hollow stem. A plurality of apertures in the cup prevent a vacuum from being drawn within the cup when the open end is brought into contact with a fetal scalp, in the absence of a sheet liner adapted to fit within the interior of the cup, engage its open edge, and having a central hole adapted to communicate with the hollow stem so that a vacuum may be drawn within the liner. The liner surface preferably includes a foil layer and covers the apertures so that the vacuum may be drawn.