Isolated spontaneous abortions may result from certain viruses—most notably cytomegalovirus, herpesvirus, parvovirus, and rubella virus—or from disorders that can cause sporadic abortions or recurrent pregnancy loss (eg, chromosomal or mendelian abnormalities, luteal phase defects). Other causes include immunologic abnormalities, major trauma, and uterine abnormalities (eg, fibroids, adhesions). Most often, the cause is unknown.

Risk factors include

Age > 35 History of spontaneous abortion Cigarette smoking Use of certain drugs (eg, cocaine, alcohol, high doses of caffeine) A poorly controlled chronic disorder (eg, diabetes, hypertension, overt thyroid

disorders) in the mother

Subclinical thyroid disorders, a retroverted uterus, and minor trauma have not been shown to cause spontaneous abortions.

http://www.msdmanuals.com/professional/gynecology-and-obstetrics/abnormalities-of-pregnancy/spontaneous-abortion

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Pervaginal Bleeding

<20 weeks gestation

With Expulsion of TissueWithout Expulsion of Tissue

Non Dilatated

Cervix

Dilatated Cervix

Partial Total

Incomplete CompleteImminent Incipient

Missed