Top diagnoses. Top reasons: 1) male infertility, tied with 2) female ovulatory dysfunction, followed by 3) tubal disease (female.) 'infertility' is defined by unprotected intercourse for one year. This is based on observing 85% of normal couples conceive in that time.
Age, STDs, PCOS. The most common factor is diminished ovarian reserve (low egg numbers and quality) which is more common as women get older, but infections like chlamydia or pid, endometriosis, smoking, or ovarian surgery can all cause diminished ovarian reserve in younger women, too. Ovulation problems like pcos, tubal problems, and fibroids or polyps inside the uterus are common, as are sperm issues for the man.