Blood from the tail vein was sampled into evacuatedheparinized blood tubes (Vacutainer, Becton Dickinson,Auckland, New Zealand) on Days 0, 7, and 15 ofthe embryo flush cycles for progesterone concentrationmeasurement. Blood samples were immediately placedin ice water, centrifuged at 4°C for 15 min at 1,500× g, and aspirated plasma was stored at −20°C untilassayed.