Timing embryo transfers during assisted reproduction

In assisted reproductive technology, the timing of embryo transfer after oocyte retrieval and in vitro fertilisation is crucial. Traditionally, embryos have been transferred at the cleavage stage (two or three days post-fertilisation). However, transferring embryos at the blastocyst stage (five or six days post-fertilisation) is increasingly common.

Most studies focus on outcomes from fresh embryo transfers, often overlooking the potential of surplus frozen embryos. Additionally, the effect of transfer stage on obstetric and neonatal outcomes has been underexplored. The linked study by Cornelisse and colleagues (doi:10.1136/bmj-2024-080133) therefore represents an important advance in this area.1 Their multicentre, randomised controlled trial (n=1202) compared outcomes over 12 months after embryo transfers on day 3 or day 5, analysing all embryo transfers within that timeframe. The study’s thorough approach, which assessed cumulative live birth rates—including both fresh and frozen embryo transfers from a single oocyte retrieval—as well as obstetric complications, distinguishes it from previous research. The study found no difference in cumulative live birth rates (risk ratio 1.01, 95% confidence interval (CI) 0.84 to 1.22), whereas live birth …