Do factors relating to patients, ART, or both, affect the incidence of chromosomal mosaicism in human blastocysts?Blastocysts (n = 5718) from 1198 PGT-A cycles were biopsied between 2015 and 2021. All samples were amplified with multiple displacement amplification, and MiSeq system was used for next-generation sequencing. Mosaicism prevalence, and ART and patient factors potentially affecting mosaicism, were analysed.Among 5718 blastocysts detected, 1245 were mosaic (21.8%). Day-6 biopsied blastocysts yielded a statistically significantly higher mosaicism rate than day-5 blastocysts (24.5% versus 19.1%; OR 1.174; 95% CI 1.017 to 1.355, P = 0.028). Mosaicism rate increased as morphological score declined (excellent quality [13.2%], good quality [19.0%], average quality [22.0%] and poor quality [26.4%], P < 0.001). Compared with excellent-quality embryos, the OR of mosaicism was 1.490 (95% CI 1.069 to 2.078, P = 0.019) for good-quality, 1.751 (95% CI 1.274 to 2.407, P = 0.001) for average-quality, and 2.113 (95% CI 1.512 to 2.952, P < 0.001) for poor-quality embryos. Biopsy technicians were related to the incidence of mosaicism. One of the four technicians had a significantly higher mosaicism rate than the others (27.9%; 20.9%; 20.5%; 20.5%, respectively; P < 0.001). Parental ages, female BMI, history of infertility, sperm quality, antral follicular counts, ovarian stimulation protocols, stimulation length, total gonadotrophin dosage and number of retrieved oocytes, were not related to the incidence of mosaicism.Slow developing, poor-quality blastocysts are at higher risk of mosaicism. Biopsy technician may contribute to artificial mosaicism as an extrinsic factor.Copyright © 2022 Reproductive Healthcare Ltd. Published by Elsevier Ltd. All rights reserved.