Cryopreservation of embryos created during fresh IVF cycles provides a less expensive and time-intensive opportunity for pregnancy. Ideal endometrium thickness is the key point to optimization FET cycle. Researches show that when endometrial thickness is less than 7mm, pregnancy rate dropped significantly. Tamoxifen has been reported to be oestrogenic on the lower genital tract. Several study on intrauterine insemination or ovulation induction suggest that tamoxifen may be a promising alternative for patients with thin endometrium. Based on this ,we want to evaluate the effects of tamoxifen on endometrial thickness and pregnancy outcome in women who failed to develop an adequate endometrial thickness in previous frozen embryo transfer cycle twice or even more times and willing to have another attempt.

Objective: To investigate the effects of tamoxifen on endometrial thickness and pregnancy outcome in women with thin endometrium undergoing frozen thawed cycle.

Study design: This is a single center, randomize control clinical trial. We plan to recruit 100 patients from Mar 2017 to Dec 2019, who failed to develop an adequate endometrial thickness in previous frozen embryo transfer cycle twice or even more times and willing to have another attempt, dividing into two group, one prepare the endometrium through ovulation induction with tamoxifen and progesterone(Study Group), the other is hormone replacement cycle (control group).

Intervention: Study Group will give endometrial preparation with tamoxifen and progesterone, control group will prepared with estrogen and progesterone supplementation.

Main outcome measures: The primary outcome of the study was endometrium thickness and clinical pregnancy rate, the secondary outcomes are cycle duration, cycle cancellation rate, implantation rate, miscarriage rate, live birth rate.

Inclusion Criteria:

1. Women age ≥20 years and ≤40 years;
2. patients that have at least 1 high quality blastocysts;
3. patients who are observed endometrial thickness ≤ 7mm in the past natural cycles or hormone replacement cycles repeatedly (twice or even more);
4. patients intends to continue to attempt another frozen embryo transfer cycle.

Exclusion Criteria:

1. patients diagnosed intrauterine adhesions, uterine scarring diverticulum via Hysteroscopy or ultrasonography and have history of uterine cavity operation for the above indication;
2. patients have congenital uterine anomalies such as Gemini, the mediastinum in the uterus and so on.

Open Label