- Older maternal age; risk slowly increases from early 20s, sharp rise > 35 - qualitative oocyte problems
- Uterine factors affecting implantation: fibroids, scarring, anatomic variations→ bicornate, etc.
- Fallopian tube problems: scarring from past STIs, TOAs, hx ectopics
- Hx trauma, radiation, surgery affecting pelvic organs
- Idiopathic in up to 20%