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.

All patients starting a thawing cycle (with frozen embryos on d1-d2-d3-d5) can be included in this RTC study. Assisted hatching will be performed with a non-contact 1.48 diode laser system (MTG, Germany).

In view of insufficient clinical evidence demonstrating the value of AH at the start of our study and in view of the need for higher implantation and LBRs in our ART programme within a series of reimbursed cycles, a prospective randomized controlled trial was performed to evaluate the effect of AH, by modified quarter laser-assisted zona thinning (mQLAZT), in our cryopreservation program. The primary aim was to test the hypothesis that the IR per embryo transferred is higher after transfer of frozen/vitrified-thawed/warmed embryos with thinned ZP after mQLAZT when compared with the transfer of frozen/vitrified-thawed/warmed embryos without mQLAZT.

Inclusion Criteria:

• All patients with frozen 2pn, embryos and blastocysts who want to start an embryo thawing cycle can be included in this study.

Exclusion Criteria:

• Patients participating in the embryo reception program are excluded.