Uterine geometry and IUCD design.

The endometrial cavity is an isosceles trapezoid that changes in shape with the phase of the cycle. The distance from the fundus to where the cavity narrows to 12 mm is the effective length. This and the total length of the uterine cavity can be measured with the Wing Sound. IUCD complications are more frequent if the length of a device exceeds the effective length or the width of the uterine cavity. Devices must be designed to fit the uterine cavity and to conform to physiological changes in the shape of the organ.