Many studies have demonstrated that endometrial scratch significantly increase pregnancy rate in patients undergoing in vitro fertilization and embryo transfer. However, only a limited number of studies have been conducted in frozen thawed transfer (FET) cycle. Nowadays, in FET cycles, endometrial scratch is mainly performed on two different time points: 1) luteal phase in the previous cycle; 2)early follicular phase in the current cycle. However, few studies have compared the effectiveness of those two different endometrial scratch methods performed in different timing. The aim of this study is to find the best timing of endometrial scratch in FET cycles.

Inclusion Criteria:

• To be aged younger than 40 years old;
• At least one period IVF-ET (fresh or thaw) has been done and not pregnant after at least one qualified blastocyst (more than 3BB) or two qualified embryos (more than II/6) have been transferred;
• There are qualified embryos thawed and the thaw-ET period will be taken.

Exclusion Criteria:

• Malformation of uterus: such as uterus uni-corns, uterus bi-corns, untreated uterus septus, etc.;
• History of endometrial diseases: such as intrauterine adhesion, uterine cavity tuberculosis, severe dysplasia of endometrium, etc.;
• Multiple myomata or endometrioma which causes uterus oppression;
• Multiple and recurrent endometrial polyps;
• Pelvic tuberculosis, endometriosis.