At present, the evaluation criterion of endometrial receptivity is controversial. The development of a molecular diagnostic tool, the endometrial receptivity array (ERA) for diagnosis of endometrial receptivity . But use of this test in patients with RIF has shown that the window of implantation (WOI) is displaced in only a quarter of these patients and use of a personalized embryo transfer (pET) on the day designated by ERA improves reproductive performance with higher cost .what is known to the others'population? The morphological changes observed on histology for each specific day after ovulation were described by Noyes and his colleagues in 1950 . An endometrial biopsy that shows a difference of more than 2 days between the histologic dating and actual day after ovulation is considered to be ''out of phase''. But such pET studies according to the Noyes criterion are lacking . The aim of this study is to explore the Personal frozen-thawed embryo transfer in unexplained RIF patients according to the blinded histological dating of endometrial biopsies .

Inclusion Criteria:

The inclusion criteria for RIF group were:

•unexplained repeated implantation failure (RIF) which is defined as the absence of a gestational sac on ultrasound at 5 or more weeks after embryo transfer (ET) after 3 embryo transfers with high quality embryos or after the transfer of ≥10 embryos in multiple transfers.

The inclusion criteria for control group were:

• age <35 years;
• regular menstrual cycles of 24-35 days;
• baseline follicle- stimulating hormone (FSH) < 9.0 IU/L;
• endometrial thickness ≥8.0 mm on the day of hCG administration.

Exclusion Criteria:

• uterine abnormalities (double uterus, bicornuate uterus, unicornuate uterus);
• intrauterine adhesions（moderate - severe）, endometriosis, adenomyosis, untreated hydrosalpinx, uterine fibroids (submucosal fibroids, nonmucosal fibroids >4.0 cm and/or endometrial pressure)
• history of adverse pregnancy (including spontaneous abortion, stillbirth, and fetal malformation).