In this study, we retrospectively analysed and compared the data of 1084 patients who underwent PGT-IVF. All 1084 cycles were divided into 6 groups based on the number of oocytes retrieved after ovarian stimulation. The demographic data showed that the number of oocytes collected was negatively correlated with female age. Group 6 had the oldest age (30.49± 4.75) and largest number of oocytes retrieved. The levels of basal E2, basal FSH, basal LH and AMH were significantly different among the 6 groups. The stimulation duration and total dose also showed negative correlations with the number of oocytes retrieved. These results were similar to those of a previous study[31]. Female age is a critical factor influencing hormone levels and ovarian response[32]. Therefore, group 6 had the highest AMH level (7.99± 4.31) and minimum Gn dose (1936.32± 661.35). Next, we explored the influence of the number of retrieved oocytes on embryo development. We found that the MII oocyte rate, 2PN embryo rate and blastulation rate significantly differed among the 6 groups. Group 1, group 3, and group 4 had the highest MII oocyte rate, 2PN embryo rate, and blastulation rate, respectively. The percentage of biopsied blastocysts did not significantly differ among the 6 groups (56.94% to 64.20%). Blastocyst euploidy is a critical indicator of developmental potential. The euploid embryo rate and mosaic embryo rate were significantly different; group 3 had the highest euploid blastocyst rate (43.84%), and group 2 had the highest mosaic blastocyst rate (13.18%). The number of oocytes retrieved had no linear association with embryo development. Ovarian hyperstimulation syndrome (OHSS) is an adverse event that occurs in the IVF cycle. Several previous studies have reported the rate of OHSS in the IVF cycle. Typically, in patients undergoing in vitro fertilization (IVF), the incidences of moderate and severe OHSS are estimated to be 3%-6% and 0.1%-2%, respectively[33, 34]. Abbara et al. reported that mild, moderate, and severe OHSS occurs in 33%, 8%, and 2% of IVF cycles, respectively, in Great Britain[35]. In the United States, data from the National Assisted Reproductive Surveillance System (NASS) from 2006 to 2015 showed that moderate OHSS decreased from 1.050% to 0.422%, and severe OHSS decreased from 0.383% to 0.106%[36]. In our study, the rates of mild and moderate OHSS were 0.09% (1/1084) and 0.09% (1/1084), respectively. Severe OHSS did not occur in the patients included in our study. The main reason is the wide use of the GnRH antagonist protocol[36]. The percentage of patients receiving the GnRH antagonist protocol in our study was 53.4% (data not shown).
Studies have shown that the number of oocytes retrieved did not influence the implantation rate, ectopic pregnancy rate, miscarriage rate, preterm birth rate, birth defect rate, neonatal weight or live birth rate. However, our study revealed that the number of retrieved oocytes has a significant influence on the clinical pregnancy rate and cumulative birth rate. The clinical pregnancy rate increased with increasing number of retrieved oocytes, and we observed a plateau when the number of retrieved oocytes was ≥26 . We also found that the cumulative birth rate was positively correlated with the number of oocytes retrieved, and no plateau was observed for the cumulative birth rate (Fig 1). However, a mild increase of 2.12% was detected for ≥26 oocytes (Table 3). On the other hand, the miscarriage rate reached a maximum (20.0%) when the number of oocytes was ≥26 (Table 3).
In their previous study, Steward et al. reported that the LB rate increased up to 15 oocytes and then plateaued (0–5: 17%, 6–10: 31.7%; 11–15: 39.3%; 16–20: 42.7%; 21–25: 43.8%; and >25 oocytes: 41.8%). However, the rate of OHSS became much more clinically significant after 15 oocytes (0-5: 0.09%; 6-10: 0.37%; 11-15: 0.93%; 16-20: 1.67%; 21-25: 3.03%; and >25 oocytes: 6.34%). Therefore, the retrieval of >15 oocytes significantly increases OHSS risk without improving the LB rate[21]. Similarly, Magnusson et al reported that at approximately 18-20 oocytes, the cumulative delivery rate per aspiration decreased, and the incidence of severe OHSS increased more rapidly[13]. Drakopoulos et al. also reported that the number of oocytes retrieved does not affect the LBR in the fresh cycle; the greater the oocyte yield is, the greater the probability of achieving a live birth after the utilization of all cryopreserved embryos[37]. However, some studies have reached the opposite conclusion: a greater number of retrieved oocytes improves the cumulative LBR without impairing the primary transfer LBR. This suggests that ovarian stimulation strategies should aim to safely maximize the number of oocytes retrieved[15, 30, 38]. All the studies above still have some limitations. These studies focused only on the live birth rate and cumulative birth rate and ignored other critical characteristics. In recent years, PGT has been widely applied in ART, and many studies have proven that PGT can improve clinical outcomes in ART[39-41]. However, none of the studies above explored the correction of the number of oocytes retrieved and the clinical outcomes of the PGT cycle. In our study, we first analysed the influence of the number of oocytes retrieved on embryo quality in PGT cycles. Furthermore, we explored the correlation between the number of oocytes retrieved and clinical outcomes after FET in PGT cycles.
There are still some limitations in this study. First, because this was a retrospective study, the bias of demographic characteristics could not be avoided. Second, this study had a relatively small sample size; therefore, to more precisely evaluate the influence of different numbers of oocytes retrieved for PGT, further studies with larger sample sizes are needed.