OBJECTIVE: Our purpose was to examine the urinary excretion of metabolites of prostacyclin, 6-keto-prostaglandin F_1α and 2,3-dinor-6-keto-prostaglandin F_1α, and thromboxane A₂, thromboxane B₂ and 2,3-dinor-thromboxane B₂, in early twin and singleton pregnancies resulting from in vitro fertilization–embryo transfer. STUDY DESIGN: Overnight urine samples were obtained from 23 women at regular intervals for 20 weeks after embryo transfer. Prostanoids were measured by high-pressure liquid chromatography–radioimmunoassay. RESULTS: All prostanoids, most markedly 6-keto-prostaglandin F_1α and 2,3-dinor-6-keto-prostaglandin F_1α, increased in both singleton and twin pregnancies. This resulted in an increased ratio of prostacyclin over thromboxane A₂ both in the dinor compounds (from 1.3 to 2.8) and the total metabolites (from 1.7 to 3.5). No marked differences were observed between twin and singleton pregnancies. CONCLUSIONS: Twin and singleton pregnancies show a similar increase in the prostacyclin to thromboxane A₂ ratio in the first half of pregnancy. (AM J OBSTET GYNECOL 1996;174:1614-7.)