The successful hatching process is a prerequisite for implantation. Freezing/thawing cycles can impair the hatching process by introducing changes in the composition of the zona pellucida. The purpose of this study is to test the hypothesis that the implantation rate per embryo and the clinical pregnancy rate per embryo transfer is higher after embryo transfer of frozen-thawed embryos with opened or thinned ZP after assisted hatching when compared to embryo transfer of frozen-thawed embryos without assisted hatching.