Just after calving, the uterus is an ideal environment for bacterial growth. During the first week postpartum, bacterial contamination occurs in up to 90 percent of cows. Whether or not full-blown metritis will develop depends on three factors:
The number and virulence of the bacteria present
The condition of the uterus

The strength of the cow’s natural defense mechanisms Metritis is commonly associated with uterine contamination from Arcanobacterium pyogenes, either alone or in conjunction with other pathogenic organisms:
Fusobacterium necrophorum
Bacteroides spp.
Escherichia coli These routine occurrences can overwhelm the fresh cow’s defense system and increase the chances of metritis:
Twins
Dead calf
Difficulty calving
Improper calving assistance
Milk fever