The identification of predictors of ART outcomes is paramount for proper treatment and patient counselling [12, 13]. Modifiable parameters such as Gn time can aid in the active management of a gonadotropin stimulation cycle.
There are few studies on this subject, and conclusions of these studies are inconsistent. Papri SARKAR et al. concluded that optimal ovarian response (as judged by the number of oocytes retrieved, number of mature oocytes, number of normally fertilized zygotes 24 hours post-retrieval, number of 6–8 cell embryos 72 hours post-retrieval, and number of embryos attaining the blastocyst stage) is associated with a stimulation duration of at least 9 days [8]. However, in their study, AMH and AFC were significantly higher in patients with cycle length > 8 days, so it is possible that the analysis was confounded by underlying reproductive issues such as ovarian reserve. TEVFIK YOLDEMIR et al. [14] proposed that ovarian stimulation cycles that are shorter than 9 days produce more embryos with more than 10% fragmentation on post-retrieval day 3. However, Brie Alport et al. [15] stated that the length of the stimulation phase is not predictive of embryo development outcomes (fertilization rate, cleavage rate or blastocyst rate). Shuo Huang et al. also showed that the differences in fertilization rate and high-quality embryo rate between patients with Gn time ≤ 7 days and controls were not statistically significant [16]. Recently, Le Tien Hsu et al. compared the results of 6–7 days of ovarian stimulation and 9–10 days of stimulation in patients aged 40–44 years and found a significantly lower number of oocytes, mature metaphase II oocytes, and 2PN zygotes in the shorter duration group; however, they observed no significant difference in the number of transferable and good-quality embryos between the two groups [17].
However, most of these studies included a wide range of ovarian stimulation protocols and were not well controlled for confounding factors. Despite these limitations, these studies inspired our research to evaluate the safe lower limit of ovarian stimulation duration.
Oocyte quality plays a crucial role in determining embryo competence and therefore ART results. Complete maturation of the oocyte determines oocyte quality [18, 19]. Therefore, obtaining mature oocytes is the key to obtaining a good treatment outcome. The complete maturation of oocytes requires both nuclear maturation and cytoplasmic maturation. Nuclear maturation consists of germinal vesicle breakdown, resumption of meiosis, extrusion of the first polar body, and arrest of the oocyte at the metaphase II stage [20]. Extensive changes in protein synthesis and posttranslational modifications in the cytoplasm take place concomitantly with nuclear maturation. Both nuclear and cytoplasmic maturation play important roles in the success of fertilization and subsequent development [21]. Nuclear maturation of oocytes can be judged from a static morphological appearance. However, the cytoplasmic maturation of oocytes needs to be assessed according to both static morphological changes and dynamic changes in oocytes in vitro [22]. Therefore, we chose oocyte retrieval rate, oocyte maturation rate, normal fertilization rate and normal cleavage rate as initial indicators of oocyte developmental competence.
Our study findings indicated that oocyte retrieval rate, oocyte maturation rate, normal fertilization rate and normal cleavage rate were not affected by shortened Gn duration in populations stratified by ovarian reserve. Although there were differences in baseline characteristics and ovarian stimulation-related factors among groups, the results remained robust after adjustment for these potential confounding factors using generalized linear model analysis. The findings of this study were consistent with those of previous studies in which shorter gonadotropin stimulation duration was found to have no association with decreased oocyte development competence [15, 16]. These data may be useful when counselling patients who are concerned about a shorter stimulation phase and may be helpful to ease the anxiety associated with the ART process. Moreover, these data indicate that IVF therapies should not be cancelled due to a short stimulation phase.
A possible reason for the lack of association between Gn time and oocyte developmental competence is that oocyte competence depends on the ovaries’ ability to develop follicles of the appropriate size, not the speed at which the follicles reach this size. The duration of gonadotropin treatment is tailored to the needs of oocyte development. However, prolonged ovarian stimulation may induce the appearance of SERa and thus reduce oocyte developmental potential. It is also possible that there are additional fundamental mechanisms involved that are yet to be discovered.
Traditionally, in an IVF cycle, the hCG trigger is administered once several follicles achieve a diameter of at least 16–18 mm and a simultaneous rise in serum estradiol has occurred [23, 24]. Clearly, it is not desirable to resolve this question by randomizing women to receive different lengths of stimulation, as this would risk oocytes being either premature or postmature at the time of retrieval. Therefore, a prospective study is not practical, and this retrospective study has value.
Our research was rigorously designed. Yihua Lin et al. retrospectively analysed 20,939 cycles and found that primary infertility, duration of infertility, BMI, previous cycles of ART, number of oocytes retrieved and protocol were factors that affected oocyte maturation rate [25]. To eliminate the influence of the above mentioned confounding factors, we included only women undergoing their first GnRH antagonist protocol cycle to study the effect of a shortened duration of ovarian stimulation on oocyte developmental competence. In addition, oocyte quality depends primarily on age. Ovarian reserve function generally declines with age, but there are also cases in which ovarian function does not reflect patient age. Therefore, the patients were grouped according to ovarian reserve function as a robust control for potential underlying or intrinsic confounding factors. We also excluded individuals with chromosomal abnormalities, endocrine abnormalities, and endometriosis to avoid these confounders. In addition, studies have shown that sperm quality can influence fertilization rates, embryo morphology and implantation rates; this is widely referred to as the paternal effect [26, 27]. Low sperm concentration and poor sperm motility can also impact on preimplantation embryo quality [28]. We also took these impacts into account in our study.
Our study is the first to analyse groups of patients with different degrees of ovarian reserve separately, which has important clinical practical value. However, several limitations should also be noted. First, the sample size of the study was limited, which reduced its statistical power. With the continuous development of IVF technology, many ovulation induction protocols have gradually emerged. The GnRH-A protocol has been used in China since 2014 and has become the mainstream program in recent years. Its advantages are that the treatment course is simplified and the treatment period is shortened. Therefore, it is in line with our research goal of shortened ovarian stimulation duration. In addition, the role of AMH in assessing ovarian reserve is increasingly recognized [29]. In 2019, our center fully carried out AMH testing for patients. Therefore, we included the population who visited our center since January 2019 as the research objects. This made our study more rigorous than other studies but it also limited our sample size accordingly. Second, selection bias in the data analysis is inevitable in a retrospective study. On the other hand we assessed oocytes developmental competence only by cells morphology, rather than molecular experiments. It may be necessary to cooperate with multiple centres and combine basic research to expand the research, and ultimately better guide clinical work in the future.