This prospective observational cohort study investigates the role of the microbiome and metabolome in couples undergoing in vitro fertilization. Biological samples will be collected from both female and male partners, including semen, vaginal, and blood samples, to characterize microbial composition and metabolic profiles. The study aims to examine how these biological profiles are associated with reproductive outcomes such as fertilization, pregnancy, miscarriage, and live birth. Participants will be followed longitudinally, and reproductive and obstetric outcomes will be obtained through linkage with national health registers.