In the field of assisted reproduction, transvaginal ultrasonography is often used to examine endometrial thickness, morphology and blood flow status to predict uterine receptivity . However, many issues, such as time of ultrasonography examination and its value in predicting IVF-ET outcome remain uncertain.
The time of choice for transvaginal ultrasonography varied in the past studies. Much research was based on records of ultrasonographic blood flow after the day of hCG injection or on the day of embryo(s) transfer. Because of the reports on increased impedance of uterine arteries after hCG injection and its influence on the predictive value of ultrasonography , we believe conducting ultrasonography before hCG allows physicians to make decisions on the time of hCG injection or treatment to improve uterine artery blood flow . Therefore, in our study, ultrasonography was conducted on the day of hCG injection.
Many researchers believe that an endometrial thickness during ovulation that is below a specific value will reduce the likelihood of pregnancy to almost zero. Past reports of this thickness ranged between 5 and 8 mm. The research by Issacs et al found the ideal range of endometrial thickness for implantation between 9 and 11 mm . The relationship between endometrial morphology and IVF-ET outcomes is also uncertain. Jarvela et al  considered that endometrial thickness and morphology related to IVF outcomes. In their study on patients receiving gonadotropin injections in an IVF-ET cycle, 44.8% of the patients with a 3-line pattern before hCG injection became pregnant; and 80% of the patient with a 3-line pattern on the day of ovum collection became pregnant. However, research by Ng et al reported no relationship between endometrial thickness, morphology and pregnancy outcomes . And in a recent study, endometrium pattern, endometrium thickness, and end-diastolic blood flow were shown to be the most effective combination for evaluation of uterine receptivity . Our study ascertained that endometrial thickness and morphology had no relationship with IVF-ET outcomes, because in our study, those whose endometrium thickness was less than 7 mm were not accepted for transferring their embryos.
Early colour Doppler ultrasonography is mainly used to assess endometrial receptivity through measuring uterine blood flow. Our study, however, found no association between uterine arterial blood flow and pregnancy outcome, as describe by Engmann et al  and we suggest that uterine artery S/D, RI and PI could not be used alone to predict endometrial receptivity.
Does endometrial blood flow represent the receptivity of the endometrium? Related studies found that the border area between the myometrium and endometrum, a thin low-echo layer, plays an important role in endometrial receptivity . The endometrial flow blood detectability rate was rather low in past years because of the limited detection technology of that time. Now, with the advance of ultrasonography, colour Doppler energy imaging has been used in endometrial blood flow assessment. Colour Doppler energy imaging is a technology based on the total integral of energy frequency spectrum. It visualizes blood flow with the energy of moving reflectors and enjoys the advantages of high sensitivity to slow blood flow, while being less dependent on angles and providing a less cluttered image.
The detection rate of endometrial blood flow in our study was 93.3%, similar to relevant reports . The condition and existence of endometrial blood flow showed direct association with pregnancy outcomes and implantation rate. In this study, only one patient without endometrial blood flow had a biochemical pregnancy, and none of the patients without blood flow had clinical pregnancy. Detection rates of endometrial blood flow in the groups of embryo diapause, miscarriage and ectopic pregnancy were significantly lower than that in the intrauterine live fetus group, suggesting an association between the condition of endometrial blood flow and embryo implantation and development. Our findings also showed that the pregnancy rate and implantation rate of the patients with detected endometrial and subendometrial blood flow were significantly higher than those with only subendometrial blood flow detection or those without detected blood flow, while the miscarriage rate in the patients with detected endometrial and subendometrial blood flow was significantly lower. The endometrial thickness and the speed of uterine blood flow had no impact on pregnancy outcomes. These findings support the association between endometrial blood flow and endometrial receptivity.
In this prospective study on 182 patients undergoing IVF-ET, the data has good comparability. We found that patients with detected endometrial blood flow had a higher pregnancy and implantation rate. Therefore, blood supply and distribution of the endometrium had a strong association with the possibility for embryo implantation and development, indicating the value of detecting the existence of endometrial blood flow in predicting IVF-ET outcomes. Research  on increased pregnancy rates and better IVF-ET outcomes with improved endometrial blood flow support our points. In another study, endometrial volume and 3D power Doppler indices were shown to be statistically significant in predicting the cycle outcome when one grade 1 or no grade 1 embryos are transferred, and thus thought it could be helpful data in a single-embryo transfer policy . As in our study, we also found a significant increasing multiple pregnancy rate in those who had better endometrial blood flow. And in a recent paper, Ng et al  discuss the relationship of endometrial blood flow between those who with a thin (less than or equal to 8 mm) endometrium and in those with a low volume (less than or equal to 2.5 ml) endometrium. It was found that 3D power Doppler flow indices of the endometrial and subendometrial regions were significantly lower in patients with a low volume endometrium compared with those with a normal volume endometrium. Endometrial and subendometrial vascularity measured by 3D power Doppler ultrasound was significantly lower (P less than or equal to 0.003) in patients with a low volume endometrium, but not in those with a thin endometrium. But the 3-D Doppler technique was not used in our study. It requires further study to explore the potential benefit of improving IVF-ET outcomes by intervening in circumstances of poor endometrial blood flow, or freezing embryos until endometrial blood flow is good enough for IVF-ET.