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Successful embryo implantation involves complex interactions between the embryo and the endometrium. Suboptimal endometrial receptivity account for almost one-third of implantation failures. Especially when we have high-quality embryos with developmental potential, how to achieve clinical pregnancy by improving endometrial receptivity is crucial. Endometrial receptivity have been the focus of extensive research for decades, since 1937 . Recent researches identified a large variety of endometrial receptivity markers associated with clinical pregnant outcomes . The markers were evaluated by ultrasound, endometrial biopsy, hysteroscopy and endometrial fluid aspirate. Among them, the most famous molecular diagnostic tool was the endometrial receptivity array (ERA), which was proposed in 2011 . So, women with recurrent implantation failure may achieve pregnancy by undergo personalized embryo transfer (pET) according to the receptive status as identified by ERA. This concept is still need to be proven by powerful searches . Because it is expensive,better chance for pregnancy is not guaranteed, invasive and take a long time to report, it is not suitable for all embryo transfer patients. in recent years, 3D color Doppler ultrasound technology has developed rapidly, with higher and higher resolution, and we can also measure parameters that could not be measured before. Second, transvaginal ultrasound is non-invasive. The ultrasound measurement takes a short time, and embryo transfer can be performed in the measurement cycle. Third, ultrasound measurement is simple and inexpensive, making it easy to generalize.

The endometrial receptivity markers evaluated by ultrasound included endometrial thickness, endometrial echo, endometrial wave-like activity and endometrial blood flow. Endometrial thickness was one of the most commonly investigated markers evaluated by ultrasound. the current routine of our reproductive center is not to perform embryo transfer when the endometrial thickness is less than 5 mm . studies have also shown that after excluding endometrial polyps or other uterin pathologies like myoma, adenomyosis , abnormal uterine shapes like septums / t shape / y shape , endometrial adhesions , embryos can be transferred without hesitation.

With the advancement of ultrasound detection technology, we can use 3D ultrasound to measure the volume of the endometrium. Compared with the endometrial thickness of a single plane, the endometrial volume can better reflect the overall state of the endometrium.

Most studies showed that triple line pattern of endometrium is associated with higher clinical pregnancy rates. The clinical pregnancy outcome also confirmed that the patients with clear endometrial central echogenic line had a significantly higher clinical pregnancy rate than its opposite The clinical pregnancy rate was also higher in patients with homogeneous echo in the endometrial functional layer than its opposite. When we observed the endometrium under ultrasound for 3 to 5 minutes, we will find that the endometrium is moving, not static. We call this endometrial peristalsis. women who conceived have lower endometrial peristalsis . It seems like that endometrial peristalsis was unfavorable for embryo implantation.

Endometrial blood flow is essential to embryo implantation. Measurement of endometrial blood flow using 3D power Doppler in the field of reproductive medicine and their role in predicting IVF cycle outcome has attract a lot of attention across the world in recent years. Most studies suggested that blood flow to the endometrium helps improve clinical pregnancy rates Some authors .speculate that fertility stress might affect pregnancy outcomes by reducing endometrial and subendometrial blood flow.

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