For patients in the study group, in the cycle immediately preceding the scheduled FET treatment, an endometrial biopsy would be arranged on day 21-23 of the menstrual cycles and they will be instructed to use non-hormonal means of contraception during that cycle. For patient in the control group, a similar procedure would be performed on the same timing during the preceding cycles without entering the uterine cavity, ie endometrial biopsy catheter entering the endocervical canal without entering the endometrial cavity. All patients will then proceed to FET treatment in the next cycle as scheduled.