Abstract Study question Does localization of diffuse adenomyosis impact reproductive outcomes after the frozen embryo transfer (FET)? Summary answer Diffuse adenomyotic lesions involving the junctional zone (JZ) have a greater negative impact on reproductive outcomes than diffuse adenomyosis of the outer myometrium (OM). What is known already Adenomyosis is associated with higher miscarriage rate, significantly lower live-birth (LB) rate, and increased risk for pregnancy-related complications. However, the impact of adenomyosis on IVF outcomes remains unclear. Adenomyosis is often treated as a homogenous disease and studies on the effects of different types of adenomyosis on reproductive outcomes have yielded inconsistent results, due to a lack of standardized diagnostic criteria and a lack of agreement on the phenotypic classification of the disease based on their severity or location. There is a lack of studies considering the effects of the severity or location of adenomyosis on reproductive outcomes after FET. Study design, size, duration This prospective cohort study was conducted at a tertiary-care hospital between January 2019 and December 2022. A total of 585 infertile women undergoing the first FET cycle were recruited. The study population included 368 women with diffuse adenomyosis where 201 women had diffuse adenomyosis of JZ and, 167 women had diffuse adenomyosis of OM. 217 women with male infertility were taken as controls. Participants/materials, setting, methods Adenomyosis was diagnosed with 2D-TVS using MUSA criteria where patients with two or more features and diffuse adenomyosis were included. Patients with diffuse adenomyosis were further divided based on the localization of adenomyotic lesions in OM or JZ. All the patients underwent FET-cycle. Pregnancy outcomes and complications were compared between different groups-those with diffuse adenomyosis of JZ, those with diffuse adenomyosis of OM, and controls. Adenomyosis patients as one group were also compared with controls. Main results and the role of chance The pregnancy rate was significantly lower in women with diffuse adenomyosis of JZ (26.37%) compared to diffuse adenomyosis of OM (47.9%) (OR:0.39, 95% CI 0.25-0.60; P < 0.0001). Similarly, the clinical pregnancy rate was also lower in diffuse adenomyosis with JZ involvement (23.38%) compared to women with diffuse adenomyosis of OM (42.51%) (OR:0.41, 95% CI 0.26-0.65; P = 0.0001). However, the biochemical pregnancy and miscarriage rates were comparable between the two adenomyosis groups. The LB rate was significantly lower in patients with JZ-involvement (16.42%) compared to women with OM-involvement (25.75%) (OR:0.57, 95% CI 0.34-0.94; P = 0.029). When all the adenomyosis patients were compared with the controls as one group, pregnancy rates were similar, but the miscarriage rate was significantly higher, and the LB rate was significantly lower in adenomyosis (P < 0.05). When the individual groups with adenomyosis were compared with controls, clinical pregnancy, and LB were comparable with the controls (P > 0.05) in women with OM involvement. However, when JZ was involved, the differences were significant (P < 0.05), highlighting the adverse impact of JZ involvement on IVF outcome. Pregnancy complications were comparable between the adenomyosis groups; however, there was a significantly higher incidence of gestational hypertension, IUGR, and pre-term labor for adenomyosis patients compared to the control (P < 0.05). Limitations, reasons for caution In the absence of a universally accepted diagnostic modality and classification system for adenomyosis, this study used ultrasound due to its lower cost and easy availability. The sample size of this study is limited to 368 patients from a single centre, larger multicentric studies are needed to make definitive conclusions. Wider implications of the findings Localizing adenomyotic lesions before starting IVF, may help in planning treatment strategies and provide adequate counseling on reproductive outcomes specific to each type. Pregnant women with adenomyosis should be managed carefully as high-risk pregnancies, considering the possible serious obstetric complications. Trial registration number CTRI/2019/01/016919