Objective: To compare obstetric and perinatal outcomes of singleton births after assisted reproductive technology (ART) with blastocyst transfer (days 5 to 6) versus nonblastocyst transfer (days 2 to 4). Design: Retrospective cohort study. Setting: Monash IVF. Patient(s): 4,202 women who conceived using in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) between 2004 and 2009. Intervention(s): Records analysis of fresh and frozen-thawed embryo transfers resulting in singleton births of at least 20 weeksa?? gestation. Main Outcome Measure(s): Perinatal outcomes: preterm birth, low birthweight, very low birthweight, small for gestational age, large for gestational age, preeclampsia, antepartum hemorrhage, placental abruption, placenta previa, and postpartum hemorrhage; and covariates: maternal age, year of birth of the baby, private health insurance status, maternal body mass index, smoking status, parity, gender of baby, and variations in treatment procedures. Result(s): Multivariate analysis found no statistically significant difference between transfers on days 5 and 6 and days 2 and 4 for all maternal and perinatal outcomes. There were modest increases in the adjusted odds ratios for preeclampsia (adjusted odds ratio 1.72, 99 confidence interval 0.93a??3.20) and placenta previa (1.65, 0.92a??2.98). Conclusion(s): Obstetric and perinatal outcomes after blastocyst transfer on days 5 to 6 are similar when compared with embryo cleavage-stage transfers on days 2 to 4.