Pregnancy outcome of vitrification-cryopreserved human blastocyst transfer: a multi-factorial assessment

Abstract

To evaluate the implantation of thawed vitrification-cryopreserved blastocysts and examine the differences in the clinical and embryonic characteristics between the groups that succeeded and failed conception. Prospective observational study. All blastocysts selectively vitrified was at least above grade B for both inner cell mass (several loosely grouped cells) and trophectoderm (fewer cells forming a loose epithelium) before freezing. Each vitrified blastocysts had been also scored by the Z-score system at the pronuclear stage 16–18 hours after IVF or ICSI. A total of 205 vitrification-frozen blastocysts were thawed in 81 transfer cycles in 73 patients. Among thaw-transfer cycles, 64cycles in 59 patients who had had surplus blastocysts vitrified for cryopreservation, and the other 17 cycles in 14 patients who had suffered severe ovarian hyperstimulation syndrome that required cancellation of the fresh transfer had all blastocysts (non-surplus) vitrified. The endometrium was prepared naturally, by clomiphene citrate-stimulation, and hormone replacement in 41, 30, and 10 cycles, respectively. After thawing, 193 blastocysts were successfully recovered with a rate of 94.1%. Among 64 transfer cycles with initial freezing surplus blastocysts, 23 (35.9%) established clinical pregnancies. There were 9 (52.9%) pregnancies in 17 cycles with initial freezing non-surplus blastocysts. The pregnancy rate was significantly lower in transfer of thawed surplus blastocysts than that of thawed non-surplus blastocysts. Totally, 32 (39.5%) clinical pregnancies with 50 intra-uterine gestational sacs occurred with an implantation rate of 25.9%. Comparison between pregnancy and non-pregnancy groups showed no differences in maternal age, the distribution of the Z1 zygotes, original total number of fresh blastocysts or fresh good-quality blastocysts, storage time of vitrified blastocysts, blastocyst recovery rate, and the method of endometrium preparation. The pregnancy rate for non-surplus blastocyst transfer (52.9%) was similar to fresh blastocyst transfer group (199 pregnancies in 394 cycles, 50.5%) during the study period. The blastocyst with its quality grade above BB is suitable for vitrification. The pregnancy outcome of thawed non-surplus blastocysts transfer is compatible with that of fresh blastocysts.