What causes infertility in most women?

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 Pelvic Inflammatory Disease (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. 40-50% of couples with infertility have a sperm issue, which may be the only problem or occur together with issues for the female partner.