They can be very diverse, from one or both partners.
In women, they are related to ovulation problems, the patency of the fallopian tubes (previous inflammatory processes that led to the closure of the tubes, the development of endometriosis with tubal occlusions, previous surgeries with the development of adhesions, etc.) or pathologies of the uterine cavity (the formation of adenomyosis that disrupts implantation, polyps, chronic endometritis, micropolyposis, adhesions, partial or complete Asherman's syndrome) or pathologies of the shape of the uterus (large fibroids or submucosal fibroids that deform the uterine cavity and make it unsuitable for implantation or carrying a pregnancy to term).
In men, it is related to problems in sperm production and altered spermogram values, or genetic defects in the structure of sperm that prevent binding with the egg cell or its activation. All of this cannot be easily determined because there are still no tests for some of the disorders. Sometimes even the best spermograms have a defect in one of the proteins in their membrane and cannot bind to the egg cell or activate it in terms of fertilization and division. The only way to determine this is through the IVF procedure when we see that in the classic IVF method fertilization did not occur but rather TFF (total fertilization failure), which gives us an answer as to why the couple did not achieve pregnancy spontaneously and that, for example, the ICSI fertilization method is suitable for that couple. There are many similar cases.
In 15% of couples, the cause of infertility cannot be detected by existing diagnostic methods, and they are classified as idiopathic infertility (of unknown etiology).