This study assessed the development of sibling embryos to blastocyst as a prognostic factor for patients undergoing embryo transfer at day 3. A retrospective analysis of a clinical and embryology database including 353 patients who underwent 393 cycles of intracytoplasmic sperm injection with day-3 embryo transfer and excess embryos, maintained in culture until day 5, was performed. Cycles were divided into group A and group B (with and without blastocyst formation, respectively). Age and basal FSH were similar in both groups. Statistically significant differences in clinical pregnancy rates (55.8% versus 40.6%; P = 0.0031), live birth rates (50.0% versus 37.2%; P = 0.012) and implantation rates (34.2% versus 23.7%; P = 0.0035) were observed in groups A and B, respectively. Odds ratios showed women from group A had 1.85- and 1.68-times the odds of patients from group B of achieving clinical pregnancy and a live birth, respectively. Cumulative live birth rate for group A, after one cycle of vitrified-warmed blastocyst transfer, was 66.4%. The development of sibling embryos to blastocyst is a prognostic factor for the outcome of the cycle in which transfer is performed at day 3 and provides valuable information about the prognosis of subsequent cycles. (C) 2013, Reproductive Healthcare Ltd. Published by Elsevier Ltd. All rights reserved.