Embryonic and neonatal outcomes following double vitrification/thawing: a systematic review and meta-analysis

This systematic review and meta-analysis aimed to evaluate the impact of double vitrification/thawing (DVT) versus single vitrification/thawing (SVT) on key embryonic and neonatal outcomes.

Information sources included systematic search in PubMed, Scopus, and Cochrane databases up to September 7, 2024. Data from each qualifying study were extracted by two reviewers using a standardized electronic data gathering form.

Mantel-Haenszel odds ratio (MHOR) and mean difference (MD) with 95% confidence intervals (CI) were calculated using both fixed and random-effects models. Subgroup analyses were based on biopsy status, number of biopsy rounds, extended culture between rounds of vitrification, and embryo transfer strategy.

A total of 35 studies involving 46,749 embryo transfer cycles were included. After excluding studies that used slow freezing, 28 studies were included in the meta-analyses. The findings indicated that DVT is associated with significant reductions in cryosurvival rates (MHOR: 0.4; CI: 0.3 to 0.8; P < 0.01), biochemical pregnancy (MHOR: 0.7; CI: 0.6 to 0.8; P < 0.01), clinical pregnancy (MHOR: 0.7; CI: 0.5 to 0.8; P < 0.01), and live birth rates (MHOR: 0.6; CI: 0.5 to 0.7; P < 0.01). Additionally, there was a significant increase in the miscarriage rate (MHOR: 1.4; CI: 1.2 to 1.7; P < 0.01). No significant differences were found in neonatal outcomes.

Poor-quality evidence suggests that the transfer of double-vitrified embryos might be associated with significantly lower rates of cryosurvival, pregnancy, and live births; however, it does not appear to affect neonatal outcomes such as birth weight and gestational age at birth. Given the small sample size in some subgroups, the high risk of selection, confounding and missing data biases, and the high level of heterogeneity for some outcomes, these findings should be interpreted cautiously.