Laser-assisted hatching is associated with reduced re-expansion of vitrified-thawed blastocysts and has no significant effect on embryo implantation.

Background: Studies have shown that vitrified-thawed blastocyst re-expansion capacity is a good predictor of implantation. However, whether assisted hatching (AH) influences re-expansion is currently unstudied. Also, whether AH improves subsequent implantation rate remains highly uncertain.

Objectives: To investigate the impact of AH on re-expansion and subsequent implantation in vitrified-thawed blastocysts transfer cycles.

Method: The absolute initial single vitrified-thawed blastocyst cycles of patients between August 2019 and April 2024 in our center were included in this retrospective cohort study, totaling 4637 cycles. Grouping was performed according to laser-AH or not. Stratified analyses according to different trophoblastic ectoderm (TE) grades were applied (TE were categorized into three different grades (A-C) according to their number and cohesiveness), with specific focus on blastocysts with TE grade of C. Subgroup analyses were then carried out based on blastocyst stage (Day5 or Day6), in which AH and Non-AH were compared separately. Multifactorial regression analyses were performed on the main outcomes to clarify the effect of laser-AH.

Results: There were no differences in pregnancy outcomes between AH group and Non-AH group, though the blastocyst stage proportions differed. Subgroup analysis based on blastocyst stage still revealed no statistically significant differences in pregnancy outcomes regarding AH or not (both in Day5 and Day6 blastocysts); while AH group had a lower re-expansion rate than Non-AH group in Day6 blastocysts (78.9% vs. 84.0%, P = 0.006). Multifactorial regression showed that AH had no effect on biochemical pregnancy rate in all cycles (aOR: 1.064, 95% CI: 0.938-1.206, P = 0.337), but increased the probability of implantation in TE grade = C cycles (aOR: 1.340, 95% CI: 1.017-1.766, P = 0.038). In the binary regression analysis on re-expansion rate, AH presented a negative effect both in all cycles and in TE grade = C cycles (all cycles: aOR: 0.774, 95% CI: 0.646-0.827, P = 0.005; TE = C cycles: aOR: 0.688, 95% CI: 0.481-0.984, P = 0.040).

Conclusion: Laser-AH negatively affects the ability of vitrified-thawed blastocysts to re-expand. Laser-AH had no significant effect on implantation in all blastocysts. AH may only be beneficial for the implantation of blastocysts with TE grade C.

Clinical trial number: Not applicable.