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Objective: To determine the clinical factors associated with blastocyst development and pregnancy. Design: Evaluation of a prospectively collected IVF database. Setting: An academic IVF practice. Patient(s): Couples (n = 529) undergoing their first IVF cycle who qualified for extended embryo culture (more than six zygotes) over the past 5 years. Intervention(s): Seven or eight zygotes were cultured for 5 days, assessed for quality, and then transferred with excess zygotes cryopreserved at the pronuclear stage. Main Outcome Measure(s): Clinical predictors of blastocyst development and pregnancy. Result(s): The mean blastocyst development rate was 49.8%, with a median number of total and good-quality blastocysts of 4 and 2, respectively. Clinical factors that were significantly associated with good-quality blastocyst formation were younger female age, increased parity, standard insemination, and lower doses of gonadotropins. Clinical factors that were significantly associated with successful pregnancy were younger female age, higher antral follicle counts, greater numbers of total and good-/excellent-quality blastocysts, and absence of male factor infertility. Conclusion(s): Several clinical factors are associated with the development of good-quality blastocysts after extended embryo culture and successful pregnancy outcome. These patient and cycle characteristics may be very useful in selecting the best candidates for extended embryo culture and single blastocyst transfers, thus optimizing outcomes while reducing the risks associated with multiple pregnancies. [Copyright &y& Elsevier]