This paper presents an evaluation of the effect of needle geometry on the strength of a tether made using a barbed suture inserted into phantom tissue using a unique device. This tether is designed to secure an intrauterine device (IUD) to uterine fundus, with the aim of improving retention of IUDs inserted in the immediate postpartum period. A factorial experiment was designed to evaluate the effect of needle geometry on tether strength. Tether strength was characterized by the peak retention force of a suture subjected to a uniaxial tensile load. Experiments were performed using phantom tissue. Two needle geometries and three suture sizes were evaluated. Sutures deposited in phantom tissue with the helical needle had up to 132% increase in retention forces compared to sutures inserted with a straight needle, with more advantage at greater active length. The helical needle provides increased suture retention force and is a suitable tether delivery mechanism for this application.