Spontaneous abortion, which is the loss of a pregnancy without outside intervention before 20 weeks' gestation, affects up to 20 percent of recognized pregnancies. Spontaneous abortion can be subdivided into threatened abortion, inevitable abortion, incomplete abortion, missed abortion, septic abortion, complete abortion, and recurrent spontaneous abortion. Ultrasonography is helpful in the diagnosis of spontaneous abortion, but other testing may be needed if an ectopic pregnancy cannot be ruled out. Chromosomal abnormalities are causative in approximately 50 percent of spontaneous abortions; multiple other factors also may play a role. Traditional treatment consisting of surgical evacuation of the uterus remains the treatment of choice in unstable patients. Recent studies suggest that expectant or medical management is appropriate in selected patients. Patients with a completed spontaneous abortion rarely require medical or surgical intervention. For women with incomplete spontaneous abortion, expectant management for up to two weeks usually is successful, and medical therapy provides little additional benefit. When patients are allowed to choose between treatment options, a large percentage will choose expectant management. Expectant management of missed spontaneous abortion has variable success rates, but medical therapy with intravaginal misoprostol has an 80 percent success rate. Physicians should be aware of psychologic issues that patients and their partners face after completing a spontaneous abortion. Women are at increased risk for significant depression and anxiety for up to one year after spontaneous abortion. Counseling to address feelings of guilt, the grief process, and how to cope with friends and family should be provided.