摘要：Objectives: To study the potential of embryo transfer after 3, 4 or 5 days of embryo culture under the German embryo protection law according to which only a maximum of three zygotes are allowed to be cultured for embryo transfer. Study design: In a prospective study, 273 patients with assisted reproductive treatment were randomly allocated for transfer on days 3, 4 or 5. Pregnancy and implantation rates were evaluated in regard to day of transfer and results were compared by Chi-square or ANOvA test. Results: Out of 234 transfer cycles, 79 were performed on day 3, 76 on day 4 and 79 on day 5. Pregnancy and implantation rates were 41.8%/27.1%for transfer on day 3, 27.6%/14.1%for day 4 transfer and 16.5%/8.8%for transfer on day 5. These results were significantly different for pregnancy rates on day 3 versus day 5 (P < 0.001) and for implantation rates on day 3 versus day 4 (P < 0.005) and day 3 versus day 5 (P < 0.001). Conclusions: These findings suggest that extended embryo culture is not beneficial when the option for embryo selection at later stages of development is not available.