A foldable one-way valve prosthesis having an open state allowing a fluid to pass therethrough, and a closed state, preventing or at least reducing a reverse fluid flow therethrough, the valve prosthesis comprising: - a closed contour base made of a relatively rigid material; - a plurality of leaflets made of a relatively flexible material; - each leaflet having: an upstream edge at which it is at least indirectly attached to the base, the upstream edge extending between two upstream vertices; a downstream edge (11down) extending between two downstream vertices, (15d, 16d) the downstream edge having a folding point dividing the downstream edge into a long section extending between the folding point and a first of said downstream vertices, and a short section extending between the folding point and a second of said downstream vertices; a pair of side edges (15, 16) oriented transversely to the upstream and downstream edges, each side edge extending between opposite upstream and downstream vertices of the respective upstream and downstream edges; and a folding line extending between the folding point and a point on the upstream edge constituted by or disposed adjacent to the upstream vertex of the side edge, whose downstream vertex is said first downstream vertex; said leaflets being attached to each other along their common side edges with a possibility of pivoting relative to each other about the common side edge when the leaflets are folded along their folding lines causing the short section of the downstream edge to be angled with respect to the long section thereof, so as to bring the valve prosthesis into said closed state, in which all said folding lines form a central area, which is substantially smaller than the area bounded by the upstream edges. - a closed contour base made of a relatively rigid material; - a plurality of leaflets made of a relatively flexible material; - each leaflet having: an upstream edge at which it is at least indirectly attached to the base, the upstream edge extending between two upstream vertices; a downstream edge (11down) extending between two downstream vertices, (15d, 16d) the downstream edge having a folding point dividing the downstream edge into a long section extending between the folding point and a first of said downstream vertices, and a short section extending between the folding point and a second of said downstream vertices; a pair of side edges (15, 16) oriented transversely to the upstream and downstream edges, each side edge extending between opposite upstream and downstream vertices of the respective upstream and downstream edges; and a folding line extending between the folding point and a point on the upstream edge constituted by or disposed adjacent to the upstream vertex of the side edge, whose downstream vertex is said first downstream vertex; said leaflets being attached to each other along their common side edges with a possibility of pivoting relative to each other about the common side edge when the leaflets are folded along their folding lines causing the short section of the downstream edge to be angled with respect to the long section thereof, so as to bring the valve prosthesis into said closed state, in which all said folding lines form a central area, which is substantially smaller than the area bounded by the upstream edges. an upstream edge at which it is at least indirectly attached to the base, the upstream edge extending between two upstream vertices; a downstream edge (11down) extending between two downstream vertices, (15d, 16d) the downstream edge having a folding point dividing the downstream edge into a long section extending between the folding point and a first of said downstream vertices, and a short section extending between the folding point and a second of said downstream vertices; a pair of side edges (15, 16) oriented transversely to the upstream and downstream edges, each side edge extending between opposite upstream and downstream vertices of the respective upstream and downstream edges; and a folding line extending between the folding point and a point on the upstream edge constituted by or disposed adjacent to the upstream vertex of the side edge, whose downstream vertex is said first downstream vertex; said leaflets being attached to each other along their common side edges with a possibility of pivoting relative to each other about the common side edge when the leaflets are folded along their folding lines causing the short section of the downstream edge to be angled with respect to the long section thereof, so as to bring the valve prosthesis into said closed state, in which all said folding lines form a central area, which is substantially smaller than the area bounded by the upstream edges.