To determine the relationship between prior obstetrical history and gestational age at delivery in a twin pregnancy. Retrospective cohort study using theUnited States Society for Assisted Reproductive Technology Clinic Outcomes Reporting System database. Clinic-based data. Patients undergoing invitro fertilization (IVF) in the United States with live delivery of twins. None. The main outcome measures are median gestational age at delivery and rate of preterm delivery (before 37 weeks). The median gestational age at delivery of IVF-conceived twins was 36.3 (interquartile rate 34.4, 37.6) weeks for nulliparous women, 35.9 (34.0, 37.1) weeks for parous women with a prior preterm birth, and 36.7 (35.1, 37.7) weeks for parous women without a prior preterm birth. The rate of preterm delivery was 61% for nulliparous women, 70% for parous women with a prior preterm birth, and 55% for parous women without a prior preterm birth. Parous women without a history of preterm delivery had lower rates of preterm delivery in a subsequent twin pregnancy than nulliparous women. Nulliparous women had lower rates of preterm delivery compared with parous women with a history of preterm delivery.