In this study of 9773 fresh embryo transfer cycles, we found that endometrial thickness after HCG trigger is not invariable; endometrial thickness decreased in 2616 cycles and not decreased in 7157 cycles (including cycles in which endometrial thickness did not change and cycles in which endometrial thickness increased). We found that the pregnancy rates were higher in the decreasing group. ETC was an independent risk factor for CPR and OPR. This conclusion is consistent with that of Jigal Haas's study.
L. Jokubkiene studied the changes in endometrial blood flow in 14 volunteers during the natural cycle and found that during the follicular phase, the sub-endometrial blood vessel index increased with follicular growth and that the thickness and volume of the endometrium increased rapidly. The sub-endometrial blood vessel index decreased to the lowest point 2 days after ovulation and then increased again during the luteal phase. This finding seems to explain the decrease in endometrial thickness in some patients during the stimulation cycle between the HCG day and embryo transfer day. We hypothesized that this endometrial change, which is highly similar to that in the natural cycle, might lead to better receptivity of embryos. Sarani SA proposed that 1 to 5 days after ovulation is the period of the menstrual cycle when endometrial vascularization is at its lowest, and it is also the period when endometrial receptivity is thought to be at its maximum in terms of hypoxia. It has been demonstrated in animal studies that near-atmospheric oxygen concentrations reduce embryo viability, and compromise embryo development and that oxygen tension in the uterus is lowest during the implantation period. Popovici RM suggested that endometrial hypoxia stimulates the production of vascular endothelial growth factor (VEGF) in endometrial stromal cells. Tsuzuki T reported that hypoxic stress stimulates the generation of VEGF through hypoxia-inducing factor-1 . The increase in VEGF in turn regulates the angiogenesis of the endometrium and stimulates further growth of the endometrium. In a single-center retrospective control study, Zhiqin Bu found that an increased endometrial thickness after progesterone administration was associated with better pregnancy outcomes for thawed blastocyst cycles . The possible reason for this conflicting result is that all of our research objects were cleavage-stage embryo transfer, with the transfer period being in the early luteal phase, in contrast to that of the blastocyst transfer, which is closer to the middle luteal phase. In addition, the effects of high estrogen levels on the endometrium during stimulation cycles may have led to different results.
We assume that the transformation of the endometrium from the hyperplasia stage to the secretion stage is accompanied by a decrease in vascularization degree and uterine thickness, which leads to a decrease in endometrial oxygen concentration. Hypoxic stress stimulates the expression of VEGF through hypoxia-inducing factor-1, which leads to an increase in the endometrial vascularization degree and further stimulates the endometrium to thicken again. The reason why this change is not obvious on ultrasound is that endometrial thickness in the luteal stage tends to vary within a smaller range. As we found in our study, the endometrial thickness in both groups changed by approximately − 1.482 ± 0.76 and 0.963 ± 1.03 mm, and there was no significant change in endometrial thickness in some cases under ultrasound examination. Even so, different patterns of changes in endometrial thickness within a small range were associated with different pregnancy outcomes.
In addition, in our study, we found that the endometrium continued to thicken in some patients from HCG day to embryo transfer day, which may reflect a relatively deficient progesterone effect, leading to inadequate endometrial densification. According to Usadi et al., this is independent of the actual circulating progesterone concentration; whether serum progesterone levels are normal or abnormally low, secretory endometrial development is similar. Some scholars believe that this may indicate the presence of progesterone receptor defects or endometrial resistance in some infertile women, and related causes include overexpression of BCL-6 and SIRT-1, chronic endometrial inflammation, progesterone receptor gene polymorphisms, altered microRNA expression, and epigenetic modifications to progesterone receptors. Another theory is that the ratio of estrogen to progesterone plays a role, with too much estrogen causing the endometrium to continue to grow. This conjecture may also explain why the endometrial receptivity of fresh embryo transplantation during the ovulatory cycle is lower than that during freeze-thaw embryo transplantation. This may be because the high estrogen level during the IVF-ET cycle leads to an imbalance in the estrogen-progesterone ratio, which leads to the failure of endometrial compression.
The advantage of this study lies in its large sample size. The measured endometrial thickness on the HCG day represents the end of endometrial hyperplasia. Since this was a retrospective study, we considered the possibility of bias in the measurement of endometrial thickness among different operators, so we had three staff members at our center perform vaginal ultrasounds on the HCG trigger day and embryo transfer day. At the same time, we conducted logistic regression to adjust for a series of confounding factors that may affect pregnancy outcomes, such as age, infertility type, infertility reason, estradiol level on trigger day, duration of ovarian stimulation, endometrial thickness on HCG trigger day, and endometrial thickness on transfer day, so the statistical results were highly reliable. We found that endometrial thickness on the trigger day was out of the equation when factors that may affect pregnancy outcomes were included in the logistic regression equation, which showed that endometrial thickness on the transfer day and ETC were still risk factors for pregnancy outcomes. This is consistent with previous studies showing that endometrial thickness is an important factor affecting pregnancy outcome.
The study's weaknesses include its retrospective nature and the many confounding factors, though we were able to adjust for these factors and analyzed them. In our center, all embryos were frozen when more than 20 oocytes were retrieved or ovarian hyperstimulation syndrome occurred. Since the number of patients with blastocyst transplantation during the egg retrieval cycle is very small in our center, this population could not be analyzed as a single group. We will consider the study of the relevant population in a future experimental design. At the same time, it remains to be seen whether this hypothesis applies to freeze-thaw embryo transfer.