P-162 Embryo transfer impact: a comprehensive national cohort analysis comparing maternal outcomes across varied embryo stages in fresh and frozen transfers
Authors: Chang C  T , Tsai E  M , Chuang H  Y , Hsu I  L , Weng S  F 
Published: July 2024

Study question

What is the ideal choice between fresh and frozen embryos transfer, and should the transfer be done at the cleavage or blastocyst stage?

Summary answer

This study highlights the clinical pregnancy and live birth superiority of frozen embryo transfer in blastocysts but not in cleavage stage embryos.

What is known already

Frozen embryo transfer not only improves reproductive outcomes by increasing the chances of live birth and clinical pregnancy but also enhances safety by reducing the risks of OHSS and multiple pregnancies. Due to the consistent findings, there has been a growing discussion in recent years about whether it is advisable to adopt elective frozen embryo transfer as the standard practice.

Study design, size, duration

In this retrospective cohort study conducted in Taiwan, a comprehensive dataset from the national assisted reproductive technology (ART) database was utilized, covering the period from January 1st, 2013, to December 31st, 2017. The final cohort study encompassed eligible 51762 female participants who had undergone ART and embryo transfer. Pregnancy outcomes and maternal complications during the follow-up were assessed using the population registry dataset from January 1st, 2013, to December 31st, 2018.

Participants/materials, setting, methods

The study categorizes cases into groups based on whether they underwent fresh or frozen embryo transfers, with additional subgroups based on cleavage stage and blastocyst stage transfers. Exposure variables include clinical pregnancy rate, live birth rate, ovarian hyperstimulation syndrome (OHSS), pregnancy-induced hypertension, gestational diabetes mellitus (DM), placenta previa, placental abruption, and preterm premature rupture of membranes (PPROM).

Main results and the role of chance

For pregnancy outcomes, frozen blastocyst transfers exhibited higher rates of clinical pregnancy (OR 1.33, 95%CI 1.25-1.42) and live births (OR 1.27, 95%CI 1.19-1.36) when compared to fresh blastocyst transfers. However, frozen cleavage stage transfers had lower rates of clinical pregnancy (OR 0.73, 95%CI 0.69-0.77) and live births (OR 0.79, 95%CI 0.74-0.84) compared to fresh cleavage stage transfers. For maternal complications, frozen embryo transfers were associated with reduced risks of OHSS (OR 0.02, 95%CI 0.02-0.03 for blastocyst and OR 0.05, 95%CI 0.03-0.09 for cleavage stage) compared to fresh embryo transfers, but were linked to a higher risk of pregnancy-induced hypertension (OR 1.43, 95%CI 1.19-1.73 for blastocyst and OR 1.23, 95%CI 1.29-1.82 for cleavage stage). No significant differences were observed in the incidence of gestational DM and PPROM among the various transfer groups.

Limitations, reasons for caution

Our retrospective study design may not offer the same level of rigor as a randomized controlled trial. Furthermore, the reproductive database did not provide detailed information on specific endometrial preparation methods (such as hormone replacement therapy or natural cycle), which could potentially influence implantation outcomes.

Wider implications of the findings

The freeze-all strategy may not be suitable for universal application. When embryos can develop to the blastocyst stage, frozen embryo transfer is a favorable choice, but embryos can only develop to the cleavage stage, fresh embryo transfer becomes a more reasonable option.

Trial registration number

KMUHIRB-E(I)-20210222