A prospective randomized study to assess the benefit of partial zona pellucida digestion before frozen-thawed embryo transfers.

BACKGROUND Assisted hatching (AH) in fresh embryo transfer (ET) could be associated with increased implantation rates. However, very few prospective randomized studies have specifically addressed the issue of AH during frozen-thawed embryo transfers (FET) cycles, those that have reported controversial results. The aim of this study was to evaluate the benefit of an enzymatic zona pellucida treatment of frozen-thawed embryos before transfer. METHODS This was a prospective study including 125 non-donor FET cycles from 125 infertile couples. FETs were randomly allocated into AH group (n = 61, embryos pretreated with pronase 5 IU/ml for 1 min at 37 degrees C) or control group (n = 64, untreated embryos). Zona pellucida thickness was measured for each transferred embryo. The main outcome parameters were clinical pregnancy and implantation rates. RESULTS The two groups were comparable regarding mean women's age, duration and indications of infertility, IVF outcome after fresh ETs, numbers and quality of fresh and frozen embryos, frozen-thawed embryo survival rates and blastomeres survival indexes. Despite a statistically significant decrease of zona pellucida thickness after pronase treatment [(mean +/- SD) 18.5 +/- 2.25 versus 14.5 +/- 2.75 microm; P < 0.0001], implantation (9.6 versus 9.2%) and clinical pregnancy rates (18.0 versus 17.2%) were not statistically different after FETs, with a similar mean number of embryos transferred between AH and control groups, respectively. CONCLUSION Within the constraints of our protocol, partial enzymatic digestion of zona pellucida by pronase was not related with any benefit of the FET outcome especially concerning the implantation ability of frozen-thawed embryos.