HORMONAL CAUSES: the follicles can not grow or oocyte release can not take place (ovulation).
TUBE PROBLEMS: uterine tubes can be damaged or obstructed, so that oocyte can not meet the spermatozoon.
ENDOMETRIOSIS: uterine mucosa (endometrium) spreads and damages adjacent tissues involved in the reproduction process. Excessive density of the cervical mucus prevents the spermatozoon from passing.
Presence of antibodies and/or antisperm in feminine genital secretion.
Unsuccessful implant of embryo in the uterus.