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Objective: To examine rates and risk factors for spontaneous abortion among pregnancies conceived using assisted reproductive technology (ART).Methods: Subjects were 62,228 clinical pregnancies resulting from ART procedures initiated in 1996-1998 in US clinics. Spontaneous abortion was based on ART clinic report and was defined as loss of the entire pregnancy. Spontaneous abortion rates for ART pregnancies were compared with spontaneous abortion rates from the National Survey of Family Growth, a population-based survey of US women 15-44 years.Results: The spontaneous abortion rate among ART pregnancies was 14.7%. This was similar to rates among pregnancies reported in the National Survey of Family Growth. Among pregnancies conceived with the patient's oocytes and freshly fertilized embryos, the spontaneous abortion risk ranged from 10.1% among women 20-29 years to 39.3% among women older than 43. Spontaneous abortion risk among pregnancies conceived with donor eggs was 13.1% with little variation by patient age. Spontaneous abortion risk was increased for pregnancies conceived with frozen and thawed embryos and decreased among multiple-gestation pregnancies. Spontaneous abortion risk was increased among women reporting previous spontaneous abortions and ART attempts, and among women who used clomiphene or zygote intrafallopian transfer. Pregnancies conceived by young women, but gestated by a surrogate, were at increased risk for spontaneous abortion in comparison with young women who gestated their own pregnancies.Conclusion: These findings suggest that ART does not pose a risk for spontaneous abortion. Factors related to oocyte or embryo quality are of primary importance in assessing spontaneous abortion risk. [ABSTRACT FROM AUTHOR]