Fertility disorders of the mare can be caused by bacterial colonisation of the reproductive organs. During natural covering, these bacteria can then lead to an infection of the stallion. For good fertility, uterus and cervix must be free of pathogenic or conditionally pathogenic agents.
You can read more about the breeding soundness examination and hormonal pregnancy diagnosis in the article by Dr Anton Heusinger and Dr. Antje Wöckener:
Breeding soundness examination and hormonal pregnancy diagnosis in horses
