['To evaluate the ability to predict pregnancy outcomes of single-blastocyst transfers by measuring the ratio of inner cell mass (ICM) diameter to blastocyst diameter using time-lapse images.', 'Retrospective cohort study.', 'University-affiliated medical center.', 'One hundred twenty-seven women undergoing a total of 129 blastocyst transfers with intracytoplasmic sperm injection.', 'Embryo monitoring by time-lapse microscopy.', 'The ratio of ICM diameter to blastocyst diameter in single-blastocyst transfers and clinical pregnancy rates.', 'In phase I of the study, 63 women underwent 65 single blastocyst transfers that resulted in 25 pregnancies (40% of the women). The successfully implanted blastocysts had an average ICM/blastocyst diameter ratio of 0.487 ± 0.086, whereas the average ICM/blastocyst ratio of nonimplanted blastocysts was significantly lower (0.337 ± 0.086). The live-birth rate was 29% (18/63). In phase II, 64 single-blastocyst transfers were performed in 64 women. The ICM/blastocyst diameter ratio was measured, and blastocysts with the highest ratios were chosen for transfer. Forty-three women (67%) with an average ICM/blastocyst diameter ratio of 0.46 achieved pregnancy, and 36 of the 43 pregnancies (84%) resulted in the delivery of a healthy baby. In the 21 women (33%) who failed to achieve pregnancy, the average ICM/blastocyst ratio was 0.45. The resultant positive predictive value was 74%, and the negative predictive value was 70%.'] Is the ratio between inner cell mass diameter and blastocyst diameter correlated with successful pregnancy outcomes of single blastocyst transfers?