Chlamydia, gonorrhea, acute salpingitis, acute cervicitis, or tuberculosis may cause endometritis. Predisposing conditions may include prolonged rupture of membranes during pregnancy, pelvic inflammatory disease, childbirth, or miscarriage. Risk factors include any sexually transmitted infection; multiple sex partners; and conditions or procedures, such as conization or cauterization of the cervix, that alter or destroy cervical mucus, allowing bacteria to ascend into the cervical cavity. Additionally, any procedure that risks transfer of contaminated cervical mucus into the endometrial cavity by an instrument, such as a biopsy curette or an irrigation catheter, or by tubal insufflation, abortion, or recent intrauterine device insertion, increases the risk of endometritis. Chronic endometritis may be caused by retained products of conception after delivery or elective abortion.

Endometritis is generally caused by more than one microorganism. A polymicrobial mixture of aerobic and anaerobic organisms is usually found and may include gram-positive cocci such as Streptococcus agalactiae, Streptococcus viridans, Streptococcus faecalis, Staphylococcus aureus, and Staphylococcus epidermidis; gram-negative organisms such as Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter aerogenes, Gardnerella vaginalis, and Neisseria gonorrhoeae; Chlamydia trachomatis and mycoplasmas such as Ureaplasma urealyticum; and Bacteroides species (B. bivius is most common), Peptococcus species, Peptostreptococcus species, and Fusobacterium species.