Zona-free blastocyst transfers have significantly lower pregnancy and live birth rates compared to zona-intact blastocysts.

Purpose: To assess whether zona-free blastocysts are associated with reduced pregnancy and live birth rates in single frozen embryo transfers (FETs), given their increasing use following trophectoderm biopsy for preimplantation genetic testing (PGT).

Materials and methods: This retrospective case-control study analyzed FET cycles performed at a single center between January 2015 and December 2023. Inclusion criteria were single day 5 or 6 blastocyst FETs; cycles involving two or more embryos were excluded. A total of 353 FETs with zona-free blastocysts were matched 1:1 with zona-intact blastocyst transfers based on PGT status, patient age at oocyte retrieval, and embryo quality. Clinical outcomes included positive hCG, clinical pregnancy, biochemical pregnancy loss, clinical pregnancy loss, and live birth rates. Statistical analyses were conducted using appropriate parametric or non-parametric tests.

Results: 2,810 single blastocyst FETs were performed, and 353 (12.6%) utilized zona-free blastocysts. Baseline characteristics, including patient age, BMI, endometrial thickness, and infertility diagnosis, were comparable between groups. Transfers involving zona-free blastocysts were associated with significantly lower rates of positive hCG, clinical pregnancy, and live birth (all p < 0.0001). While biochemical and clinical pregnancy loss rates did not differ significantly, the overall pregnancy loss rate was significantly higher in the zona-free group compared to zona-intact (p = 0.04).

Conclusions: Single blastocyst FETs involving zona-free blastocysts are associated with significantly lower pregnancy and live birth rates compared to zona-intact blastocysts.