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The use of single-embryo transfer (SET) effectively reduces the twinning rate while maintaining high pregnancy rates in appropriately selected patients undergoing in vitro fertilization (IVF). Although SET in unselected patients eliminates twins, it may significantly lower pregnancy rates compared with double-embryo transfer. Some clinicians are reluctant to use SET in their practice despite clear evidence of its benefit because of concern over reduced pregnancy rates. There is variation among various centers in prognostic criteria used to determine patients who would benefit most from SET. To date, only 5 studies have evaluated prognostic factors for SET; none were performed in the United States.This study was designed to identify additional patient, cycle, and retrieval characteristics associated with embryo implantation and live birth rate in a select group of favorable prognosis patients undergoing SET. The cohort was composed of patients treated at an academic outpatient center enrolled into a prospectively collected IVF database. All patient cycles meeting criteria for SET between 2004 and 2010 were included. The primary outcome measures were clinical pregnancy and live birth.Of the 438 patients who underwent SET, 334 (76.2%) achieved clinical pregnancy; the rate of live birth per transfer in these women was 66.8%. Factors independently associated with clinical pregnancy and live birth rate were younger female age and blastocyst expansion. At the time that oocytes were retrieved, at least 58% were mature (metaphase II); use of mature oocytes was more likely to result in a clinical pregnancy. Younger female age and use of more expanded blastocysts were associated with ongoing pregnancy. There was a negative association of a diagnosis of uterine factor with live birth. No association was found between obesity (high body mass index) and clinical pregnancy or live birth.These findings show that younger female age and blastocyst expansion are associated with clinical pregnancy and live birth in a population of women with favorable prognosis. The only infertility diagnosis that affects live birth after SET is a diagnosis of uterine factor. Obesity does not appear to negatively affect SET outcome.