The junctional zone (JZ) is an internal layer of the myometrium, with unique characteristics that allow its visualization as a hypoechogenic structure in three-dimensional (3D) ultrasound. Although traditionally evaluated by magnetic resonance imaging, 3D reconstruction offers an opportunity for reliable and easily accessible assessment. The only study that evaluated the thickness of the JZ in 3D ultrasound in the context of assisted reproductive technology (ART) found that the smaller the thickness of the JZ, the greater the rate of embryonic implantation. This project aims to evaluate the relationship between the characteristics of the ZJ in 3D ultrasound, and the clinical outcomes of ART treatments, namely in vitro fertilization cycles, intracytoplasmic sperm injection and frozen embryo transfer (FET). Additionally, the intra and inter-observer variability of the characteristics of the JZ will be determined. In a prospective and observational study with the inclusion of 200 cases, a 3D ultrasound will be performed on the day on which the final oocyte maturation is triggered or, on FET, on the day prior to the administration of progesterone. The images will be evaluated independently by two observers. The principal investigator will evaluate the images in two stages. After the quality of visualization of the JZ is classified, its thickness will be measured and described as regular, irregular or interrupted. The volume of the ZJ will be obtained by subtracting the endometrial volume from the volume of the junctional zone and the endometrium. These characteristics will be related to the clinical outcomes of each cycle: clinical pregnancy rate and abortion rate.