Objective: To evaluate the infl uence of early cleavage in embryos obtained using ICSI on the success rate of IVF - ET. Design: A prospective study. Setting: Centre of Assisted Reproduction, Department of Obstetrics and Gynecology, Medical Faculty, Palacký University, Olomouc. Methods: The rate of the fi rst cleavage of embryos was checked between 23 to 26 hours after ICSI in 115 cycles IVF - ET. „Early Cleavage“ (EC) embryos which were already cleaved were cultivated separately from the „slow“ (No Early Cleavage, NEC) embryos. After 3 days cultivation, no more than three 6- to 8-cell embryos were transferred. Only EC embryos were transferred in 43 cycles and only NEC embryos were transferred also in 43 cycles. The remaining 29 cycles, where both EC and NEC embryos had to be transferred, were not evaluated. In both groups the following entry parameters were compared: age of the patient, number of fertilized oocytes, fertilization rate (FR), number of frozen and evaluated embryos, number of EC embryos recorded and number of transferred embryos. The outcome parameters compared were: pregnancy rate (PR), number of gestational sacs with heart beats per number of transferred embryos (implantation rate, IR), abortions (AB) and the frequency of multiple pregnancy. Statistical evaluation was performed by t-test, χ2 test and by Sheffe analysis. Results: EC embryos were found in 57 % of cycles (n=66) after ICSI. Early cleavage was observed in 198 of 563 embryos (35%). Among entry parameters the only statistically signifi cant differences (P<0.05) were found in the number of retrieved oocytes as well as in the ratio of frozen embryos which were higher in the cycles with the transfer of only EC embryos. FR was the same (74%) in both types of transfer. Outcome parameters in the transfer of only EC versus NEC embryos were higher in PR (40% vs. 33%), about equal in IR (27% vs. 25%) and lower in AB (2% vs. 5%). Moreover, multiple pregnancies were less frequent in the transfer of only EC embryos: 65% of singleton pregnancy, 35% of twins and no triplet versus 36%, 57% and 7%, resp., in the transfer of only NEC embryos. The differences in outcome parameters are not statistically signifi cant (P>0.05) because of small numbers. Conclusion: The speed of the fi rst cell cleavage is a useful additional criterion for better evaluation of quality and viability of embryos.

        Key words: early cleavage“ embryos, embryo selection, embryo quality and viability, in vitro fertilization