Abstract: An obstetrical instrument for rupturing in a safe and easy manner the foetal membranes of a pregnant woman in order to release the amniotic fluid contained in these membranes. The instrument comprises an elongated tubular member preferably made of a semi-rigid plastic material, which member is adapted to be inserted into the woman's vagina until its front end is in contact with the foetal membranes to be ruptured. A piston manually actuatable by a rod extending outwardly of the tubular member through the rear end thereof is movably mounted inside this member to create, when actuated, a vacuum sufficient to draw to a certain extent the foetal membranes inside this tubular member. Piercing pins are provided inside the tubular member for rupturing the foetal membranes when these membranes are so drawn inside the member on actuation of the piston with the rod.