Question: what can cause unexplained fertility?
Answer: Anatomical abnormalities. It used to be thought that a retroverted uterus (or a tilted womb) was a cause of infertility, but this is no longer taken to be the case. ... 
Abnormal development of the follicle and of ovulation. ... 
Abnormal eggs. ... 
Trapped eggs. ... 
Luteal phase abnormalities.