The results of our study suggest that patients with high rates of blastulation are, as may be expected, inherently more likely to yield more high-quality blastocysts during IVF. These patients with high blastulation rates are not necessarily the patients with the most oocytes retrieved, which may surprise some patients who believe that high oocyte yield is associated with universally better outcomes. Improved blastulation rate and formation of high-quality embryos were not found to be associated with any particular patient or IVF cycle characteristics. This finding highlights the fact that IVF cycle outcomes, including blastulation rate and formation of high-quality embryos, are difficult to predict.
It is unknown whether infertility diagnosis may impact blastulation rates or likelihood of formation of top-quality embryos. Infertility diagnoses have in prior studies been correlated with variable obstetric outcomes after IVF. Specifically, ovulatory dysfunction and PCOS have been associated with poorer obstetric outcomes . In a large retrospective cohort study, higher blastulation rate was associated with a higher incidence of tubal and uterine factor infertility, whereas low blastulation rates were associated with a higher rate of unexplained infertility . The potential for variable IVF outcomes by infertility diagnosis is an area for further research. Of note, the percentage of cycles in which ICSI was performed was significantly higher than the percentage of cycles in which male factor infertility was listed as the diagnosis (Tables 2 and 3). The decision to perform ICSI in the DFC is made on a provider-by-provider basis, and ICSI is performed regularly for certain other clinical scenarios such as planned PGT-A, unexplained infertility (with the thought that fertilization defect may be the yet-unknown cause of infertility for a certain couple), frozen sperm, and nulliparous patients who have never had proven fertilization potential with their partner’s sperm.
IVF outcomes have been shown to be impacted by patient ethnicity, BMI, and age. Our study did not show any such association, but it is possible that the sample size was too small to detect any difference. We would expect worse blastulation rates and lower formation of top-quality embryos in patients with high BMI and who were older. In one large retrospective cohort analysis, blastulation rates were significantly lower in older patients . The oldest patients in our study did have the lowest blastulation rates, but these results did not reach statistical significance.
E2 and P4 levels on the day of ovulation trigger are routinely tracked in IVF cycles. Elevated P4 is thought to have detrimental effects on endometrial receptivity, and cycles with elevated progesterone are typically managed with a freeze-all technique rather than with fresh embryo transfer . There is some evidence to suggest that elevated progesterone also results in a decreased formation of high-quality embryos . Progesterone has been found to be an independent predictor of pregnancy in IVF cycles, with higher P4 levels being associated with a decreased likelihood of pregnancy . While higher progesterone was noted in patients with lower blastulation rates in our study, this did not reach statistical significance.
There are no prior studies to our knowledge that assess E2 levels and their association with formation of top-quality embryos. Our study did not show any such association, but it may be that there were an inadequate number of available subjects to detect any difference. Blastulation rates and blastocyst quality associated with E2 and P4 levels are an area for further investigation.
The finding of a lower number of retrieved oocytes in those patients with the higher blastulation rates (although this did not reach statistical significance) is an intriguing finding that warrants additional discussion. This finding was also reported by our group in a large retrospective analysis of 70,968 cycles, where those with the highest oocyte yield had the lowest blastulation rates . Oocytes are, of course, essential in the formation of blastocysts for successful IVF cycles, but it is increasingly being recognized that a greater number of oocytes retrieved may not necessarily be associated with the best IVF outcomes [12, 14,15,16,17,18, 21]. It has been suggested that the optimal number of oocytes retrieved in a cycle may depend on the exact stimulation protocol and that a lower number of oocytes retrieved is not necessarily correlated with poorer IVF outcomes [12, 15, 17, 22]. This data suggests that an emphasis on embryo quality over oocyte quantity may be a more appropriate strategy for the management of IVF cycles. Despite this evidence, low oocyte retrieval remains a feared outcome for patients and providers alike.
More “mild” ovarian stimulation during IVF cycles has several benefits, including decreased incidence of ovarian hyperstimulation syndrome (OHSS), decreased frequency of multiple gestations, decreased rates of aneuploidy, lower cost, and improved access to IVF treatment . Milder stimulation protocols have been associated with better implantation and pregnancy rates despite a lower number of oocytes retrieved . Multiple mechanisms to explain this paradox that less oocytes may yield improved IVF outcomes have been proposed. Elevated E2 and P4 levels seen with conventional IVF protocols may be associated with direct embryo toxicity and diminished endometrial receptivity, and as such have poorer IVF outcomes . It has been suggested that elevated E2 may disrupt folliculogenesis and oocyte maturation. Incidence of low birth weight, preeclampsia, and long-term health consequences for IVF offspring have been shown to be increased in patients with supraphysiologic E2 concentrations in early gestation . Therefore, excessive E2 levels noted in aggressive stimulation protocols are best avoided. Available evidence supports that gentler ovarian stimulation selects for the best quality follicles and creates better quality oocytes and embryos . However, it is important to note that all of the patients in our study underwent conventional controlled ovarian hyperstimulation, and the mean number of oocytes retrieved in each cohort was between 13 and 18; these oocyte yields would be difficult to achieve with minimal stimulation protocols. We also acknowledge that there were no differences in the FSH dose per blastulation cohort, indicating that high FSH dose itself may not be the culprit affecting blastulation rates. As stated above, we hypothesize that the estradiol and/or progesterone levels reached by the patient (of which high responders are at particular risk of achieving high hormonal milieu by the time of trigger) may have more of an effect on the development of embryos than the stimulation protocol. Thus, we hypothesize that if, in a minimal stimulation cycle, a patient were to achieve an adequate oocyte yield to recommend extended culture of embryos, those embryos may likely display high rates of blastulation given the more physiologic hormonal milieu at the time of oocyte maturation trigger.
Blastocyst quality is highly pertinent clinically in choosing the most ideal embryos for transfer, which are associated with significantly improved clinical pregnancy and live birth rates in IVF [2, 3, 9, 17]. Despite this, there is little data to guide reproductive endocrinologists on how to improve or predict blastulation rates and anticipated blastocyst quality. Improvement of blastulation rates and formation of high-quality embryos is an area for much future investigation in IVF research.
The strengths of this study include the availability of hormone levels and embryo quality for evaluation in our data set. This is the first study to our knowledge that investigates the association between estrogen levels, rates of blastulation, and embryo quality. Limitations of this study are inherent in its retrospective design, small sample size, and single site analysis. Also, the included cycles in this study are not representative of the overall population undergoing IVF, given that only those cycles with a good number and quality of cleavage stage embryos underwent extended culture to blastocysts.