Occurrence of smooth endoplasmic reticulum aggregates in metaphase II oocytes: relationship with stimulation protocols and outcome of ICSI and IVF cycles

STUDY QUESTION: Is there any association between the appearance of smooth endoplasmic reticulum aggregates (SERa) in oocytes and ovarian stimulation, embryological, clinical and neonatal outcomes of ICSI and IVF cycles?SUMMARY ANSWER: A suboptimal prolonged ovarian stimulation is detrimental to oocytes by inducing the occurrence of SERa, which reduces the reproductive potential of oocytes.WHAT IS KNOWN ALREADY: Controlled ovarian stimulation recruits oocytes of different qualities. Based on current evidence, it was agreed that non-homogeneous cytoplasm may represent the normal variability among oocytes rather than a dysmorphism with developmental significance. The only exception is the appearance of SERa within the ooplasm. Owing to the lack of univocal evidence in this literature about the safety of injecting oocytes with SERa and the mechanism responsible for the occurrence of SERa, this topic is still a matter of debate.STUDY DESIGN, SIZE, DURATION: A retrospective, longitudinal cohort study performed at a tertiary level public infertility center. We included 1662 cycles (180 SERa+ and 1482 SERa-) from 1129 women (age: 20-44years) who underwent IVF/ICSI treatments in 2012-2019. The SERa+ cycles had at least one SERa+ oocyte in the oocyte cohort. The SERa- cycles had morphologically unaffected oocytes.PARTICIPANTS/MATERIALS, SETTING, METHODS: We collected stimulation data and embryological, clinical, neonatal outcomes of SERa- and SERa+ cycles and oocytes.MAIN RESULTS AND THE ROLE OF CHANCE: Overall, 347 out of 12436 metaphase II oocytes (2.8%) were affected by SER. We performed only 12 transfers involving at least one SERa+ embryo. Stimulation length (P=0.002), serum progesterone (P=0.004) and follicle size (P=0.046) at trigger, number of retrieved (P=0.004) and metaphase II (P=0.0001) oocytes were significantly higher in SERa+ than SERa- cycles. Fertilization rate was significantly (P