Abstract
Study question
Does Assisted hatching (AH) improve success rates when applied to frozen embryo transfers?
Summary answer
AH does not improve implantation, ongoing pregnancy or live birth rates when applied to thawed embryos.
What is known already
Vitrification has been proven to be the most efficient technique to preserve human embryos. However, vitrification has some consequences for the embryos, zona pellucida (ZP) hardening being one of them. Multiple studies suggest the need to apply laser Assisted hatching or ZP thinning to thawed embryos in order to improve success rates. Still, there is not enough evidence to ensure the utility of AH, and considering the great variation in design between studies more evidence is needed.
Study design, size, duration
Study performed from October 2019 and January 2020. Disregarding embryos with natural Hatching and PGT-A. Embryos that, immediately after thawing, were completely expanded (trophectoderm in contact with ZP) were also excluded from the study. We applied a randomization to choose in which embryos we had to perform AH. Neither the gynecologist nor the embryologist performing the embryo transfer knew whether the embryo had AH performed or not.
Participants/materials, setting, methods
353 frozen embryo transfers of one blastocist were considered for the study, 71 excluded for expansion after thawing, 65 excluded because of PGT-A, 103 in which we performed AH (AH+) and 114 without AH (AH-). In the AH+ group we performed laser-AH of 1/3 of the ZP, avoiding to damage the trophectoderm and performing the laser shots as far away to the ICM as possible. We used Chi-square testing to assess the effects of AH.
Main results and the role of chance
We assessed all relevant clinical data parameters. No statistical differences were found in egg age, maternal age, embryo quality, nor endometrial thickness between groups. Implantation and miscarriage rates were equivalent between AH+ group (40.9%; 20.5%) and AH- group (47.4%; 18.5%).
The main outcome of this study was live birth rates. No statistical differences were found between groups (AH-= 38.6%; AH + = 30.1%; p = 03221) proving that making it easier to get out of the ZP does not affect success rates.
Analyzing the data from the excluded embryos we found no improvement on live birth rates when embryos were expanded just after thawing (38.0%; p = 0.457). As expected, PGT-A embryos yielded higher live birth rates (52.3%; p < 0,05)
Limitations, reasons for caution
Preliminary study with a small data set.
Wider implications of the findings: This study suggest that thawed embryos have the capacity to get out of the ZP regardless if AH was performed or not. Having no positive effects, AH seems to be unnecessary in this scenario.
Trial registration number
Not applicable
Growth differences after fresh and frozen embryo transfers: When do they begin?