Ultrasound has been used to detect and evaluate intrauterine device (IUD) position for some years. Different definitions of the IUD in the uterine cavity have been discussed trying to relate them to bleeding and pain complaints. Most of these definitions do not take into account the uterine wall or endometrial thickness. This is a secondary analysis of a previous cross-sectional study, in which 481 IUD users had their T-shaped IUD evaluated by transvaginal sonography. The correlation between the endometrium thickness and the IUD-myometral distance was studied, defined by the linear measurement between the upper end of the IUD and the myometrium/endometrium interface. These two measurements were highly correlated over the entire sample range (r = 0.29) and an even stronger correlation existed (r = 0.66) when only cases below the 90 percentile of the IUD-endometrial distance distribution were considered. These results suggest that the IUD position in the uterine cavity is influenced by the growth and thinning of the endometrium, and that this information should be considered when evaluating the IUD position by sonography.