Approximately 30% of pregnancies in the United States may end in miscarriage or stillbirth. Whether pregnancies conceived through assisted reproductive technology (ART) are at an increased risk of loss is inconclusive, and data on maternal age-, ART type-, and gestational age-specific risk of loss are limited. Data on 148,494 ART pregnancies conceived from 1999 through 2002 were analyzed by use of the Kaplan-Meier method to estimate risks of pregnancy loss after specified gestational ages (conditional risk) for 14 groups stratified by maternal age and ART procedure. Births, maternal deaths, and induced abortions were censored. The Kaplan-Meier estimate of total risk of pregnancy loss was 29% but ranged from 22% to 63% depending on patient age and ART procedure. By 6 weeks' gestation, 58% of all pregnancy losses occurred. Conditional risk of pregnancy loss ranged from 10% to 45% at 6 weeks' gestation and from 2% to 7% at the first trimester; it was less than 2% after 20 weeks' gestation. Results can be used to counsel ART patients and inform future research on the etiology of pregnancy loss. Alemán