STUDY QUESTION Is there is an association between follicle size and the quality of oocytes retrieved from them as judged by ability to achieve the blastocyst stage, blastocyst grades and blastocyst ploidy? SUMMARY ANSWER Although follicle size is a valuable predictor of oocyte maturity and is a significant predictor of the ability of a fertilized oocyte to become a quality blastocyst, the ploidy of each quality blastocyst is not related to the size of the follicle from which its oocyte was retrieved. WHAT IS KNOWN ALREADY It is unclear whether the oocytes within larger follicles are the best oocytes of the cohort. Although there have been studies examining follicle size in relation to embryo quality, there has been no study relating the incidence of euploidy in embryos to follicle size. STUDY DESIGN, SIZE, DURATION The purpose of this study was to examine follicle sizes and the oocytes from those follicles (and the embryos that result from those oocytes) to see if there is an association between follicle size and the quality of oocytes as judged by ability to achieve the blastocyst stage, blastocyst grades and blastocyst ploidy. Follicle sizes for oocytes were assessed both as diameters (mm) and as Z values (expressed as their size relative to the mean and standard deviation of that donor’s follicular cohort). Comparisons were made using cumulative histograms, rolling averages and receiver operator characteristic (ROC) curves and its AUC. PARTICIPANTS/MATERIALS, SETTING, METHODS Twenty-two oocyte donors (ages: 24.5 ± 3.5 years) whose recipients would use ICSI for insemination were enrolled in this study. Follicles were aspirated one-at-a-time to be certain that the aspirated oocyte was from the same follicle measured. The follicle measurement (size) was noted in the embryology records. Oocytes were cultured individually throughout their time in the embryology laboratory so that follicle sizes could be uniquely associated with each oocyte. Oocytes and embryos were analyzed according to the size of the follicle from which the oocyte was retrieved. MAIN RESULTS AND THE ROLE OF CHANCE Three hundred seventeen oocytes (96.1%) had an associated follicle size. Of the oocytes with follicle sizes, 255 (80.4%) had a polar body (MII), and 60 (18.9%) were immature: 31 (9.8%) with a visible germinal vesicle (GV stage) and 29 (9.1%) with neither a polar body nor a visible germinal vesicle (MI). The incidence of MII oocytes was significantly associated with larger follicle size using either mm (ROC’s AUC = 0.87; P < 0.0001) or Z values (ROC’s AUC = 0.86; P < 0.0001). Among MII oocytes there was no association with follicle size for the appearance of 228 oocytes with two pronuclei (2 PN). Among 2 PN’s, the development of 94 quality blastocysts that underwent trophectoderm biopsy (TE Bx) exhibited a significant association with larger follicles using either mm (ROC’s AUC = 0.59; P = 0.01) or Z values (ROC’s AUC = 0.57; P = 0.01). The use of follicle diameter as a feature to distinguish between fertilized oocytes that would ultimately become blastocysts versus those that would not become blastocysts resulted in an enrichment for blastocyst formation from 20 to 40%. Of the 94 quality blastocysts, 51 were determined by next generation sequencing (NGS) to be euploid.Although oocyte maturity and the incidence of blastocyst formation were associated with follicle size, the incidence of euploidy among biopsied blastocysts was not. Follicles measured by two different methods (mm or Z values) led to predominantly the same conclusions. LIMITATIONS, REASONS FOR CAUTION This study investigated the relationship between follicle size and measures of oocyte/embryo quality when donors were treated similarly. Therefore, this study does not investigate the effects of triggering and retrieving oocytes when the follicle cohorts are of different sizes or lead follicles are of different sizes. Although no association was found between follicle size and euploid blastocysts, the fact that blastocyst ploidy is not entirely dependent upon oocyte ploidy (e.g. aneuploidies derived from mitotic errors or from the fertilizing sperm) makes it difficult to infer the relationship between follicle diameter and oocyte ploidy. WIDER IMPLICATIONS OF THE FINDINGS It is confirmed that follicle diameter is predictive of oocyte maturity. However, once oocyte maturity is known, the diameter of the follicle from which the oocyte was retrieved is not instructive. Embryos generated through fertilization and development of the mature oocytes from any observed follicle diameter were equally likely to become euploid blastocysts. STUDY FUNDING/COMPETING INTEREST(S) This study was funded by ReproART: Georgian American Center for Reproductive Medicine. None of the authors declare any actual conflicts of interest. D.H.M. received compensation from ReproART, Biogenetics Corporation and the Sperm and Embryo Bank of New York and honoraria and travel funding from Ferring Pharmaceuticals and from Granata Bio. S.M. received compensation from Cooper Genomics and an honorarium and travel funding from Ferring Pharmaceuticals. L.C. is the founder of LTD Ovamedi, the organization that represents Cooper Genomics in Georgia, and received travel funding from the European Society for Human Reproduction and Embryology. TRIAL REGISTRATION NUMBER N/A.

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