Comparison of pregnancy outcomes after vitrification at the cleavage and blastocyst stage: a meta-analysis

This systematic review sought to evaluate the clinical outcomes of vitrification at the cleavage stage and blastocyst stage for embryo transfer in patients undergoing assisted reproductive technology (ART) treatment. We searched for related comparative studies published in the PubMed, EMBASE, and Cochrane Library databases up to July 2017. The primary outcomes were clinical pregnancy rate (CPR) and embryo implantation rate (IR). Secondary outcomes were multiple pregnancy rate (MPR), miscarriage rate (MR), live birth rate (LBR), and ongoing pregnancy rate (OPR). The Mantel-Haenszel fixed effects model and random effects model were used to analyze the summary risks ratios (RRs) with 95% confidence intervals (CIs). Eight studies with more than 6590 cycles were included in our meta-analysis. Seven studies were observational retrospective comparative studies. One was a prospective study. Overall, the current study summarizes information from 6590 vitrification warming cycles (cleavage stage n = 4594; blastocysts n = 1996). There was no difference in the primary outcome clinical pregnancy rate (RR = 0.97, 95% CI = 0.90-1.04; fixed effects model; I (2) = 21%), whereas vitrified blastocyst transfer was significantly superior to vitrified cleavage-stage embryo transfer regarding the implantation rate (RR = 0.85, 95% CI = 0.74-0.97; random effects model; I (2) = 43). Regarding the secondary outcomes, no differences were found in the multiple pregnancy rate (RR = 1.20, 95% CI = 0.79-1.82; fixed effects model; I (2) = 22), live birth rate (RR = 1.07, 95% CI = 0.98-1.16; fixed effects model; I (2) = 0), and ongoing pregnancy rate (RR = 1.01, 95% CI = 0.92-1.120; fixed effects model; I (2) = 0), whereas a higher miscarriage rate was observed with vitrified blastocyst transfer (RR = 0.65, 95% CI = 0.45-0.93; random effects model; I (2) = 23). In summary, this meta-analysis shows that vitrification at any stage has no detrimental effect on clinical outcome. Blastocyst transfer will still remain a favorable and promising option in ART. Due to the small sample evaluated in the pool of included studies, large-scale, prospective, and randomized controlled trials are required to determine if these small effects are clinically relevant.