An intrauterine device has a pair of initially flat, registering, triangle-shaped sheets of soft, rubber-like, substantially liquid-impervious, non-toxic flexible material of about the same dimensions as the undistorted human uterine cavity. The sheets are reinforced by a mat of flexible or pliable, non-stretchable fibers to prevent stretching of the sheets. The initially flat sheets are thermally preformed into a three-dimensional final shape and size and are then secured together in a marginal area forming a flat band all around the edges except for a gap at one apex. The sheets are also secured together along internal areas preferably bisecting the apices of the triangle except at said gap and except at a central location. This leaves unsecured or free zones in communication with each other and with said gap and extending approximately parallel to the sides of the triangle. In use, the unsecured zones are filled with fluid, such as liquid, introduced through the gap. This stiffens them so that, although they remain soft, they act somewhat like beams and are not readily distorted. The liquid can contain hormone crystals for slow release through a governing polymer coating.