To Mask or Not to Mask Mosaicism? The Impact of Reporting Embryo Mosaicism on Reproductive Potential

Purpose: To evaluate euploidy rates and probability of having at least one euploid embryo for transfer per cycle when mosaicism is reported compared to when it is masked.

Methods: Women age 18-46 years who underwent PGT-A with next generation sequencing of blastocyst biopsies were analyzed. When reported, mosaic embryos were classified as low-level, 20-40% mosaic, or high-level, 41-80% mosaic. When masked, low-level mosaics were categorized as euploid and high-level mosaics were considered aneuploid. Comparative analyses were performed with χ tests and t-tests.

Results: A total of 22,504 PGT-A biopsy cycles from 18,401 patients were included. These cycles were from 293 different clinics with a mean of 1.22 cycles per patient. The majority of cycles (94.8%) reported mosaicism, and only 5.2% cycles were masked. The euploidy rate was significantly lower when mosaicism was reported versus masked (38.7% v 47.4%, p < 0.0001), which remained significant for age 40 years old and younger. The mosaic reporting cohort was less likely to have at least one euploid embryo for transfer (68.8%) compared to the masked cohort (75.7%) (p < 0.0001); however, this was no longer significant after stratification by age.

Conclusion: Mosaicism reporting shows an overall expected reduction in euploidy rate. In turn, the probability of having a euploid embryo to transfer depends on clinic transfer practices and patient preference. If mosaic embryos are not transferred, we observe a reduction in probability of having an embryo for transfer. Although the magnitude of these differences is small, our data show that mosaic reporting may contribute to embryo attrition rate.