It's well known, endometritis affects reproductive outcomes. Though CD138 counting as one new diagnosis measure for endometritis, diagnostic criterion of this measure isn't consistent. And if the CD138 counting is correlation to the outcome of reproductive is not clear.

Firstly, This study aims to explore the relationship between endometrial CD138 count and pregnancy outcome following frozen embryo transfer in natural cycles. Secondly , the investigators want to find a cut-off value which is related to reproduction outcome.

Inclusion Criteria:

• age between20 and 38 years;
• regular menstrual cycles;
• body mass index (BMI) ranged from 18 to 25

Exclusion Criteria:

• uterine abnormalities;
• intrauterine adhesions（moderate - severe）, advanced endometriosis, adenomyosis, untreated hydrosalpinx, uterine fibroids (submucosal fibroids, nonmucosal fibroids >4.0 cm and/or endometrial pressure)