Effect of laser zona thinning on vitrified–warmed embryo transfer at the cleavage stage: a prospective, randomized study

The aim of this study was to determine if laser zona thinning could improve the rates of pregnancy and implantation for vitrified–warmed embryo transfer at the cleavage stage. A total of 400 vitrified–warmed embryo transfer cycles were randomly assigned to either the test group or the control group. The zona pellucida of vitrified–warmed embryos in the patients of the control group was untreated, whereas in the test group it was partially thinned by laser just before embryo transfer. In the test group, the clinical pregnancy and implantation rates were significantly lower as compared with that of the control group (28.5 versus 43.0, P = 0.002, and 11.2 versus 16.7, P = 0.004, respectively). Therefore the results of this investigation show that laser zona thinning may have an unexpected adverse effect on the rates of clinical pregnancy and implantation of vitrified–warmed embryos at the cleavage stage.