Should artificial shrinkage be performed prior to blastocyst vitrification? A systematic review of the literature and meta-analysis.

Embryo vitrification is increasingly used in IVF. Artificial shrinkage (collapse) before vitrification has been proposed to maximise blastocyst survival after warming. However, its effectiveness on blastocyst survival rate and vitrified-warmed blastocyst transfer cycle outcome remains to be confirmed. Therefore, we performed a systematic MEDLINE search according to PRISMA guidelines on all articles published up to April 2018 and related to human blastocyst collapse before vitrification using the following keywords: (i) blastocyst; (ii) collapse; (iii) artificial shrinkage; and (iv) vitrification. The following outcomes were analysed and included in the meta-analysis: (i) blastocyst survival rate after warming; (ii) implantation rate; (iii) clinical pregnancy rate; and (iv) live birth rate after vitrified-warmed blastocyst transfer (commonly named frozen-thawed blastocyst transfer). Eight articles were included. Briefly, blastocyst survival (OR 5.04, 95% CI 2.43-10.46) and clinical pregnancy rate (OR 1.87, 95% CI 1.26-2.77) were significantly higher in collapse than in control group. However, implantation rate (OR 2.50, 95% CI 0.67-9.28) and live birth rate (OR 1.35, 95% CI 0.88-2.09) were comparable in both groups. In conclusion, this systematic review and meta-analysis suggests that artificial shrinkage before blastocyst vitrification improves survival and clinical pregnancy rate, but not implantation or live birth rate. Further randomised studies are warranted to improve the level of evidence and confirm these findings.