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Abstract: Objective: To investigate the factors that might influence the success of an embryo freezing and thawing program. Method: The relationship between the pregnancy rate in 73 cycles of embryo freezing and thawing program and the following factors was analyzed: maternal age, E₂ level at the time of HCG trigger, embryo storage time, number of thawed embryos transferred, presence of sponsoring embryos and intact embryos. And the survival rate of thawed embryos with different morphology, cell stage and storage time was evaluated. Result: Tra nsfer with three or more than three thawed embryos resulted in pregnancy rates of 38.5% and 35.7%, respectively, compared with 5.3% for transfer of fewer than three embryos. The presence of sponsoring embryos and intact embryos significantly increases pregnancy rate in embryo freezing and thawing program. No other factor examined had any effect on pregnancy outcome. The survival rate of good morph ology embryos was higher than poor ones, but was not influenced by cell stage and storage time. Conclusion: embryo morphology before freezing, number of thawed embryos transferred and the presence of intact embryos are important to the outcome of embryo freezing and thawing program.

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