For patients in the study group, in the cycle immediately preceding the scheduled IVF treatment, LH surge will be determined by daily serum LH level starting from 18 days before the next expected period. LH surge is defined as an elevation of LH to 2 times the level of the average of the previous 3 days and the absolute level of the LH should be more than or equal to 20 IU/L. Women randomized into the study group will have endometrial biopsy performed by pipelle 7 days after the LH surge (LH+7) and they will be instructed to use non-hormonal means of contraception during that cycle. All patients will then proceed to IVF treatment in the next cycle as scheduled. They will receive standard ovarian stimulation according to the departmental protocol and have a maximum of two embryos replaced 2 days after the oocyte retrieval. On-going pregnancy rates between the two groups will be compared.