Despite some controversial reports, most studies, to date, strongly support the idea that a premature rise in progesterone levels on HCG trigger day during ovarian stimulation is associated with reduced implantation and pregnancy rates following fresh embryo transfers. However, almost all studies focused on the effect of significantly elevated progesterone level on the trigger day on the clinical outcomes after IVF/ICSI cycles[3-6]. This study assessed the clinical outcomes between slight elevation progesterone level 1.0-1.5ng/ml with normal progesterone level<1.0ng/ml for the first time.
The mechanism causing elevated progesterone on the trigger day during COH is unclear, which is currently believed to be related to the following factors[2,7-10]: (1)excess mature follicles; (2)high Gn dose: Bosch et al. found that for every 75 U increase in FSH, the risk of elevated progesterone on HCG days increased 1.44-fold; (3)High LH; (4)prolonged ovarian stimulation; (5) different ovulation promotion protocols: GnRH agonist protocols were most associated with extreme elevations of progesterone. In the present study, total dose of Gn, days of stimulation, E2 levels on the HCG day and number of oocytes retrieved in Group 1 were all higher than these in Group 2 (1.0<P<1.5ng/ml), consistent with the results of most of the above studies. Also, the women in Group 1(P<1.0ng/ml) had a higher BMI and there was a statistical difference between the two groups(23.49±3.54 vs 22.45±2.99, P=0.002), suggesting that women with low BMI are more likely to have elevated progesterone. It is possible that the higher the BMI, the higher the aromatase expression and, consequently, the higher the estrogen production, which could impact the production of progesterone[11].
Although the “freeze-all” strategy can avoid the effects of high progesterone levels on pregnancy outcomes, there is no uniform expert consensus on the safety and long-term prognosis of this strategy. Therefore, careful selection of the dose of ovulation stimulating drugs and reduction of the duration of stimulation time are particularly important to prevent premature elevated progesterone on HCG day in order to improve the transplantation rate of fresh cycles.
There are different threshold values defining elevated progesterone,and retrospective studies have demonstrated that progesterone levels greater than 1.5–2.0ng/mL are associated with reduced pregnancy rates and fresh embryo transfer should be cancelled[2,9,12]. Although most clinicians think progesterone level<1.5ng/ml have no negative effect on outcomes, the present study found that the clinical pregnancy outcome was poor when the serum progesterone level higher than 1.0ng/ml under early follicular phase prolonged protocol cycles, the same with another study[13].
According to previous studies, elevated progesterone may affect ART outcomes in several ways. Huang et al. found that elevated progesterone levels on HCG days had a significant negative effect on the rate of high-quality embryos(P < 0.05)[14]. However, there are some studies that take the opposite view. Based on various markers of oocyte and embryo quality, the data strongly support that there is no negative influence of premature progesterone elevation on oocyte and embryo quality[14-16]. Therefore, the conclusion is yet to be confirmed by a large prospective study. In this study, there was no significant difference in embryo quality between the two populations, and both were dominated by high-quality blastocysts.
On the other hand, elevated progesterone may affect endometrial tolerance. By examining the transcriptome profile of peri-implantation window endometrium, Liu et al. found that in endometrium with high progesterone levels on HCG day, the expression profile of genes related to natural killer cell-mediated cytotoxic pathway was significantly different compared to normal levels of endometrium[17].
Some clinicians have proposed a question: are good patient and embryo characteristics protective against the negative effect of elevated progesterone level? Studies showed that high progesterone(>1.5ng/mL) did not change the pregnancy rate in cases of hyperresponders[18,19]. In the present study, analyzed the data including maternal age (30.78±3.76, 30.94±3.87), AMH (4.21±2.92, 4.01±2.85), AFC (15.42±5.57, 14.47±5.13), Basal FSH (6.44±1.85, 6.55±1.82), number of oocytes retrieved (12.41±4.26, 13.78±4.36), suggesting that the vast majority of women in both cohorts could be classified as high-response patients with a good prognosis. However, a slight increase progesterone(>1.0ng/mL) on the day of HCG still had a negative impact on pregnancy outcome. This result may suggest that the negative impact of premature P elevation on pregnancy rates is an endometrial effect, as opposed to an oocyte effect.
Furthermore, some studies hypothesized only in cases of D3 embryo transfer, there would be interference from progesterone(P>1.5ng/mL) on the HCG day, the implantation rate would drop by 50%; the same would not occur in the transfer of blastocysts[8]. In contrast, in this study, D5 single blastocyst transfer was used in all these women, yet the high progesterone group (P>1.0ng/mL) still had lower clinical pregnancy rates and live birth rates. The results of this study suggest that on the fifth luteal day, the endometrium still has not recovered from the effects of high progesterone and its impact on pregnancy outcome still persists.
An important strength of the present study was that it is the first study to have compared the ART outcomes between women with progesterone<1.0ng/ml and progesterone>1.0 to<1.5ng/ml under an early follicular phase prolonged protocol. Also both groups used the same COS protocol and all underwent D5 single blastocyst transfer, avoiding the influence of relevant confounding factors. In addition to comparing implantation rates and early pregnancy rates, an extended comparison of the differences in live birth rates between the two groups was also performed. Another significant factor to consider in assessing any study on P level is the accuracy and precision of the assay[20]. The internal and external quality control of the assays utilized in our laboratory ensured that these standards were maintained. Weaknesses of this study include its retrospective design and small sample.