Despite the lack of a consistent definition for "mild stimulation" (MS), the International Society for Mild Approaches in Assisted Reproduction defined it as a protocol performed with gonadotropins, alone or with oral compounds, at lower doses or for a shorter duration, with the aim of achieving 2-7 oocytes. One of the strategies proposed for MS is the use of Clomiphene Citrate (CC). CC acts as a selective estrogen-receptor modulator. By blocking estrogen receptors in the hypothalamic arcuate nucleus, it increases the production of gonadotropin-releasing hormone (GnRH) and, as a result, FSH and luteinizing hormone (LH). Moreover, CC increases the pituitary sensitivity to GnRH and granulosa cell sensitivity to pituitary gonadotropins. Taking these actions into account, several protocols have adopted a combination of CC and exogenous gonadotropins with the aim of improving follicular recruitment and, therefore, ovarian response to stimulation, in patients undergoing in vitro fertilization (IVF). The available evidence has allowed for the inclusion of CC in international guidelines as a treatment option, alone or in combination with gonadotropins, equally recommended in the management of poor responders when compared to gonadotropin stimulation alone.

The concept behind MS is that, with this approach, only the healthier follicles with higher quality oocytes are allowed to grow. Proponents of this protocol state that MS reduces the risk of multiple pregnancy and ovarian hyperstimulation syndrome (OHSS), as well as patient dropout rate and treatment costs. However, evidence regarding clinical outcomes is far from consensual. The best available evidence regarding MS in predicted poor responders comes from the OPTIMIST trial, showing no difference in the cumulative live birth rates when a mild approach, using 150 IU of rFSH, was compared to an individualized protocol of 225/450 IU rFSH. However, several methodological inconsistencies have been pointed out in this randomized controlled trial. In particular, a black hole was left in the management of predicted low responders with an intermediate prognosis (antral follicle count between 8-10), taking into account the allowance for dose adjustments in the second cycle in the 150 IU group. Considering that the control group was treated with rFSH 225 IU daily, a comparison of two identical doses might have been provided.

Evidence regarding the effect of MS on embryo quality is also conflicting. Baart et al. first reported a lower aneuploidy rate following MS when compared to conventional protocols and concluded that mitotic segregation errors might increase with growing gonadotropin dosages. However, this has not been confirmed in recent studies. As for the number of good quality embryos, while previous studies have shown no difference regarding MS and conventional protocols, Vermey et al found a positive correlation between the number of retrieved oocytes and the embryo quality.

Although these previous studies provide some valuable information, the heterogeneity of the available evidence cannot be disregarded. Moreover, to the best our knowledge, the effect of the intensity of ovarian stimulation on early embryo development has not been previously described. Therefore, the investigators set out to perform this randomized controlled trial comparing the number of good quality blastocysts (GQB) and morphokinetic parameters of early embryo development in patients with a predicted suboptimal ovarian response undergoing two different intensities of ovarian stimulation, a milder (CC 50 mg/day from cycle D2-6 + rFSH 150 IU daily from D2 onwards) and a more intense approach (300 IU daily dose of rFSH starting on cycle D2).