Improved implantation rate after chemical removal of the zona pellucida.

Objective: To evaluate the implantation rate achieved after chemical removal of the zona pellucida from day 5 human in vitro-derived embryos.

Design: Prospective, randomized, controlled study.

Setting: A tertiary care infertility clinic.

Patient(s): Two hundred fifty-seven patients undergoing IVF with transfer of morulas or blastocysts on day 5 after oocyte retrieval. All patients had had at least two previous implantation failures.

Intervention(s): Chemical removal of zona pellucida by using acidic Tyrode's solution vs. no removal (controls).

Main Outcome Measures: Clinical pregnancy rate and implantation rate per transfer.

Result(s): Embryos without zona pellucida implanted at nearly twice the rate of control embryos (15.7% vs. 27.5%). The pregnancy rate was also significantly higher in the zona pellucida-free group than the control group (31.0% vs. 46.1%). Removal of zona pellucida was most effective in embryos with delayed development, which reached the morula or early cavitating stage on day 5 of in vitro culture (implantation rate, 12.1% vs. 25.7%).

Conclusion(s): Chemical removal of zona pellucida from day 5 in vitro cultured human embryos is an effective and safe method of significantly improving the implantation rate, especially of embryos with delayed development.