In frozen embryo transfer cycles, is the pregnancy potential of the embryo associated with the number of days the embryo spends in in-vitro culture before freezing?

Lay summary

In-vitro fertilisation (IVF) provides people with the chance to have a baby. However, the clinical pregnancy success rate of IVF remains quite low; around 36% each time an embryo is transferred. During IVF, the hope is for the embryo to implant and form a healthy baby following the embryo transfer. For an embryo to create a successful pregnancy, it must reach the blastocyst stage, which usually happens after 5 to 6 days of culture. However, some embryos can reach this important stage at a faster rate, after only 4 days. These faster embryos may have a better chance of creating a successful pregnancy, but research is needed to investigate this. T Blastocysts which form early on day 4 are usually frozen on that day for later use in a frozen transfer cycle. Good quality blastocysts that form on day 6 and day 7, as well as surplus blastocysts on day 5, are also often frozen. Therefore, in this project, we want to analyse the results of frozen embryo transfer cycles, using embryos frozen on day 4, compared to day 5, 6 and 7.

The aim of this project is to find out whether transferring a frozen day 4 blastocyst will result in a better pregnancy success rate. We also want to look at the chance of a patient having a problem during pregnancy, or problem with the newborn baby, after a frozen day 4 blastocyst has been used in treatment. This project will reach this aim by comparing pregnancy outcomes after frozen transfer of a day 4, 5, 6 or 7 blastocyst.

Public benefit statement

This research has the potential to improve embryo selection in IVF which could lead to better pregnancy success rates and contribute to improved outcomes for patients.