The number and quality of oocytes and embryos are important determinants of success of any ART cycle. Serum estradiol as a factor affecting these parameters has been studied and published in few instances, with conflicting results. In this study, we tried to find a correlation between the serum estradiol levels on day of trigger with mature follicles, oocytes, and mature follicles, with the aim of finding out whether estradiol levels can be a clinical tool is assessing oocyte and mature oocyte yield.
Age had a negative relation with the number of follicles (p = 0.015) and number of oocytes retrieved (p = 0.041) (Table 5). Similarly, age had a significant negative correlation with estradiol values (p = 0.021) (Table 6), both indicating diminished ovarian reserves with aging. Similar finding was seen in the study by Vaughan et al. [9], where E2 levels on day of HCG was found to be inversely proportional to patient’s age and the quantity of oocytes retrieved also declined. As maternal age increased, a linear increase in E/O ratio was observed since the number of oocytes retrieved declined. A simple estradiol level measured on day of HCG would not have brought out this finding. Study by Kara et al. [10] also showed NRO higher in age group < 36 years, so as also pregnancy. Mitwally et al. [7] commented in their study that women above 35 years are more vulnerable to high E2 levels; and also had significantly lower pregnancy rates. The negative correlation started at lower E2 levels in women above 35 years.
Obesity was associated with decreased number of mature oocytes in the present study, though not statistically significant (Table 7). This was similar to the observation in the study by Siddhartha et al. [11], where overweight women with BMI > 26 kg/m2 had significantly decreased E2 values on day of HCG. AMH had a very significant positive relation with the number of follicles, number of oocytes, and the number of mature follicles (p < 0.001). It also has a strong positive relation with the estradiol values on day of HCG (p =0.002). AFC, the other parameter analyzed had a very strong positive relation with all the three parameters—number of follicles, oocytes, and mature oocytes (p < 0.001), also, with the estradiol values (p < 0.001).
Estradiol vs NF, NRO, NMO
As estradiol increased, estradiol per follicle, oocyte, and mature oocyte increased. As estradiol increased, there was a significant increase in the number of follicles—NF (p < 0.001), number of oocytes retrieved—NRO (p < 0.001), and the number of mature oocytes—NMO (p < 0.001). Similar findings were noted in the study by Xin Li et al. [12], where NRO, NMO, and CPR increased as E2 level increased. But beyond a level of 5000 pg/ml, all these decreased. Their study revealed that E2 levels < 1000 pg/ml and > 5000 pg/ml had a negative impact on IVF outcome.
Erzincan et al. [13] also made similar observations in their study, where the NRO increased parallel to E2 levels, with no significant differences in clinical pregnancy rates (CPR) between various E2 levels. In the study by Siddharth et al. [11], the NRO and NMO were highest when estradiol was > 4000 pg/ml, as in the present study. CPR was highest in the group with estradiol between 3000 and 4000 pg/ml, suggesting that very high levels of estradiol may be detrimental for implantation. Reduced implantation and pregnancy rates when E2 > 90th centile was shown in the study by Arslan et al. [4], even when the NRO, NMO, and embryo scores were similar in various E2 groups. Similar positive correlation of E2 levels with NRO, NMO was noted in the study by Mittal et al. [14] also. The study by Kara et al. [10] also mirrored similar results with NRO parallel to E2 levels, maximum at > 4000 pg/ml. The findings of Anifandis et al. [6] observed that very low and very high E2 levels on day of HCG yielded very low and very high number of oocytes respectively, again suggesting a linear correlation. The study by Yu Ng EH et al. [3] concluded that high E2 levels in IVF cycles adversely affected implantation and pregnancy rates. They observed that embryo quality was unaffected by E2 levels and it was the adverse endometrial milieu associated with very high E2 levels that affected implantation. Frozen thawed embryos from same cycle implanted well. Almost similar observation was made by Kolibianakis et al. [2] who observed that high E2 adversely affects endometrium and therefore implantation, but had little impact on M2 oocytes and embryo quality.
However, a systematic review by Kosmas et al. [15] did not find any high-quality evidence to support or deny the value of E2 on day of HCG administration for IVF outcome. They found no positive correlation on retrospective evaluation of studies.
The finding by Kyrou et al. [5] was similar to that by Yu Ng et al. [3], the NRO, NMO, and good quality embryos increased with E2, but not pregnancy, probably due to the deleterious effect on endometrial receptivity. The results of the study by Mitwally et al. [7] mirrored that by Anifandis [6]. They found that cycles with low and high AUC-E2 values had significantly lower pregnancy rates particularly if the patient was > 35years. The study inferred that women above 35 years of age are more vulnerable to high E2 levels. Present study did not include the reproductive outcome. NRO and NMO increased parallel to estradiol levels as was found in the many of the mentioned studies.
The ESHRE guidelines for ovarian stimulation state that the addition of estradiol measurements to ultrasound monitoring was neither found superior to ultrasound monitoring alone nor does it recommend the timing of final oocyte maturation trigger based on estradiol or estradiol/follicle ratio as it was not found superior in terms of efficacy and safety than ultrasound monitoring alone [16].The present study found that as estradiol values increased, you could expect more oocytes and mature oocytes, in turn leading to a probably better ART outcome. So also as estradiol/oocyte increased, the outcome in terms of mature oocytes was poor. The present study aimed at predicting ART outcomes in term of oocyte and mature oocyte yield rather than using estradiol values as means of follicular monitoring during ART stimulation.
E2/follicle vs NRO and NMO
E2/follicle was not found to have a significant correlation with the NRO and NMO in the present study. In the study by Mittal et al. [14], however, a positive correlation was seen between E2/fol and NRO NMO, fertilized oocytes, and embryo quality. Study by Ozdegirmenci et al. [17] also showed that E2/follicle correlated positively with NRO, NMO.
E/O vs NMO
Study showed E2/O had a strong negative correlation with the NMO (p value < 0.001). This was observed in many other studies as well, suggesting that raised E2 per oocyte is probably linked with maturation of oocytes. In the study by Vaughan et al. [9], NMO was found to be lowest with E2/O < 250 and was highest when the E/O was > 2000. CPR was highest when E2/O ratio was in the range 250-750 but declined as the ratio increased. Lowest fertilization rates were measured at extremes of ratio (< 250 and > 2000) reflecting negative impact of increasing E/O on reproductive outcome. A negative correlation between E2/O and NMO and number of fertilized oocytes was also found in the study by Ozdegirmenci et al. [17]. They concluded that while E2/follicle had a positive correlation with NRO, NMO, and fertilized oocytes, but E2/O adversely affects these parameters. Mittal et al. [16] showed a similar negative correlation of E2/O and NRO, NMO commenting on the probable negative effect of high E2 levels on maturation of oocytes.
Limitations
It was a retrospective study. The effect of different protocols of IVF on estradiol was not separately considered. The study also did not include the effect of estradiol on quality of embryos and IVF outcome.
