Limitations and strengths
Considering the lack of evidence related to a shorter exposure to oestrogen in an HRT cycle, we did not have enough knowledge and evidence to directly perform a powered RCT. Therefore, a major limitation of the present study is the design as a pilot trial. As a result of its limited study population, it was underpowered to determine the superiority of one intervention over another. Instead, the purposes of the present study were to explore trends in pregnancy rates for each HRT strategy and to provide us with enough knowledge for the sample size calculation of further definitive RCTs in which a non-inferiority of the 7 days E2 priming approach could be confirmed. Although the pilot design, these results allow us to safely design a larger confirmatory RCT, exploring the safety and efficacy of a FET-HRT with the aim to reduce TTP.A vital strength of this trial is its strict inclusion criteria, such as the choice to include only single transfer of good quality blastocysts [Lastly, we performed a hormonal assessment on the day of ET to understand the possible influence of the different protocols on hormonal trends and cycle outcomes, to have a fully comprehensible understanding of the HRT protocol.