 In the Centre of Reproductive Medicine of the Brussels University Hospital, the investigators start progesterone supplementation 5 days before the transfer of a day 3 embryo. In order to mimic the natural cycle, since progesterone starts to rise 2 to 3 days before ovulation, due to the LH-stimulated production by the peripheral granulosa cells Speroff. In fresh IVF-cycles, progesterone is started from the day of oocyte retrieval. HCG administration in fresh cycles will lead to an increase in progesterone levels and therefore, endometrium will progress quickly, compared with frozen-thawed cycles which are hormone supplemented Nawroth. 