The majority of SABs are caused by chromosomal abnormalities that are incompatible with life; the majority also have autosomal trisomies. Maternal infections, such as Mycoplasma hominis, Ureaplasma urealyticum, syphilis, HIV, group B streptococci, and second trimester bacterial vaginosis, increase the risk for an SAB. Inherited disorders or abnormal embryonic development resulting from environmental factors (teratogens) may also play a role. A woman's occupation, such as a hair stylist, may also be a factor in SAB if she is exposed to teratogens. Unfavorable environmental factors also include interpersonal violence, as women who are in abusive relationships have a 50% higher chance of pregnancy loss.

Patients who are classified as habitual aborters (three or more consecutive SABs) usually have an incompetent cervix—that is, a situation in which the cervix is weak and does not stay closed to maintain the pregnancy. Another reason for habitual abortions may be antiphospholipid antibodies and polycystic ovarian disease.