The FET would be carried out as per our protocol. In short, patients attend the clinic daily from 18 days before the next expected period for the determination of serum E2 and luteinising hormone (LH) concentrations until the LH surge, which was defined as the day on which the LH level was above 20 IU/L and doubled the average of the LH levels over the past three days. For patients with irregular menstrual cycles or no ovulation demonstrated during natural cycle monitoring, clomiphene citrate (CC, Clomid, Merrell, Staines, U.K.) 50-100 mg will be given daily for five days from Days 3-7. The cycle will be monitored by blood tests from day 10 of the cycle as above. FET was performed on the third day after the LH surge. Up to two frozen-thawed embryo(s) can be transferred. No luteal phase support is used. On-going pregnancy rates, defined as viable fetuses beyond 10-12 gestational weeks, between the two groups will be compared.