BACKGROUND: The uterine junctional zone is the inner part of the myometrium. Dysfunction of the zone may underlie the pathogenesis of adenomyosis and its clinical manifestations, while biometric characteristics of the zone are currently considered as promising early diagnostic criteria for this disease. Adenomyosis has traditionally been associated with parity and intrauterine interventions, primarily in older patients. However, modern imaging tools often allow diagnosing the disease in young patients with infertility and an unburdened gynecological history. It is assumed that the detection of changes in the structure and function of the uterine junctional zone in adenomyosis can be the basis for predicting fertility outcomes and complications of pregnancy, as well as for the development of promising therapeutic strategies at the pregravid stage. AIM: The aim of this study was to assess the influence of biometric characteristics of the uterine junctional zone on pregnancy outcomes, depending on the parity and intrauterine interventions in patients with adenomyosis. MATERIALS AND METHODS: This prospective study included 102 patients aged 22–39 years old with ultrasound features of adenomyosis who were going to conceive. The patients were divided into two groups: Group 1 (n = 58) consisted of nulliparous patients with no history of previous intrauterine interventions, and Group 2 (n = 44) comprised multipara women with a history of labor and / or intrauterine interventions. Using magnetic resonance imaging, we evaluated minimal, average and maximal junctional zone thicknesses, junctional zone deferential and a ratio of junctional zone thickness to myometrium thickness. Thresholds of biometric characteristics of the uterine junctional zone for adverse pregnancy outcomes were estimated. RESULTS: The frequencies of pregnancy and retrochorial hematoma in patients of Groups 1 and 2 in the first trimester of pregnancy did not differ significantly and amounted to 43.1% and 38.6%, 13.8% and 22.7%, respectively, p > 0.05. Adverse pregnancy outcomes were diagnosed in 63.8% of patients in Group 1 and in 68.2% of patients in Group 2, p > 0.05. In Group 1, the frequency of retrochorial hematoma depended on the initial junctional zone deferential, as well as on the initial average and maximal junctional zone thicknesses, junctional zone deferentials and ratios of junctional zone thickness to myometrium thickness, which, with an adverse pregnancy outcome, were 1.7–2.5 times higher than those in patients with a favorable outcome, p > 0.05. In Group 2, adverse pregnancy outcomes were recorded with significantly higher values of average and maximal junctional zone thicknesses and junctional zone deferential. ROC curves were constructed using data of logistic regression analysis based on biometric characteristics of the uterine junctional zone to predict spontaneous abortion and infertility in patients with adenomyosis. CONCLUSIONS: Fertility outcomes in patients with adenomyosis depend on a complex of biometric characteristics of the uterine junctional zone as determined by magnetic resonance imaging.