Assisted hatching improves implantation rates on cryopreserved-thawed embryos. A randomized prospective study.

Background: Focus on the hatching process has so far been in the field of fresh embryos. Cryopreserved-thawed embryos have a lower rate of pregnancy than fresh embryos, which might be due to hardening of the zona pellucida.

Methods: During a 2 year period, a prospective randomized study enrolling 253 cryopreserved-thawed cycles was performed on day 2 embryos.