What is the optimal duration of progesterone administration before transferring a vitrified-warmed cleavage stage embryo? A randomized controlled trial. - van de Vijver, A.; Polyzos, N. P.; Van Landuyt, L.; Mackens, S.; Stoop, D.; Camus, M.; De Vos, M.; Tournaye, H.; Blockeel, C. (Human Reproduction, 2016)