The reasons for unwanted childlessness are manifold.
In general, the desire to have children by many couples has been postponed because education and work have a higher interest. The best biological age for a pregnancy is often surpassed when the desire for a child becomes stronger. In the case of the majority of the couples, one or both of the partners has a physical impairment that prevents the couple from becoming pregnant. In women, these are mainly closed fallopian tubes, endometriosis and the PCO syndrome. In men, problems with sperm production and sperm transport cause fertility problems.
Mental stress, long-term stress, heavy overweight or underweight, excessive alcohol or nicotine consumption, drugs, excessive exercise, heavy work, low-vitamin diet and high radiation exposure have negative effects on fertility. Also, previously suffered  infections, such as mumps, can reduce the quality of semen.