Rates and risk factors of oocyte immaturity: toward personalized selection for rescue in vitro maturation

To identify treatment-related factors influencing low oocyte maturation rates in standard ovarian stimulation (OS) cycles and patients' candidates for clinical studies on rescue-IVM.

Retrospective study including retrievals with ≥ 1 cumulus-oocyte-complex (COC; years: 2008-2022). The weighted-mean immaturityrate (19%) was defined in the whole dataset (N = 16,155). Variables associated with immaturity rates and warning limit (defined as weighted average + 2SD) were appraised among first retrievals with ≥ 5 COCs (N = 7962). Of the patients undergoing a first oocyte pick-up, 667 completed three retrieval cycles, enabling evaluation of the true prevalence of patients exceeding the immaturity rate warning threshold over multiple cycles.

Factors influencing immaturity rate included OS duration, trigger type (GnRH-agonist versus urinary-hCG), ovulation trigger to oocytes' denudation interval and ratio COC to follicle > 14 mm at ovulation trigger. In first retrievals with ≥ 5 COCs, the immaturity rate warning limit was 51%, occurring in 3.6% of initial retrievals, 3.8% of second retrievals, and 2.1% of third retrievals. In three consecutive retrievals, the conservative prevalence of patients exceeding this threshold once, twice, and three times was 4.4%, 0.3%, and 0.03%, respectively. Assuming all patients would have conducted three retrievals, these rates were estimated as 7.8%, 1.5%, and 0.3%. In the 667 patients who conducted three retrievals, observed rates were 7.6%, 0.9%, and 0.4%, confirming the reliability of the estimates.

Ovarian stimulation and laboratory factors impact oocyte maturation rate. An oocyte immaturity rate exceeding 51%, in patients retrieving ≥ 5 oocytes, may represent a strong inclusion criterion for future clinical studies on rescue-IVM.