The stable hydration products of the vasoconstrictory and proaggregatory thromboxane A2 and vasodilatory and antiaggregatory prostacyclin, ie, thromboxane B2 and 6-ketoprostaglandin F1a, respectively, were measured with radioimmunoassays from 67 women with normal pregnancy, preeclampsia, or other pregnancy complications with the determination of the blood flow in the umbilical vein with the ultrasound method. In addition, the maternal platelets' capacity to release thromboxane B2 was studied. No relation was seen between these prostanoids and the umbilical blood flow and/or pregnancy complications. Moreover, the concentrations of the prostanoids were similar in women with high (161.1 ± 6.8 mL/minutes/kg of fetal weight, mean±SE, N=33) and low (50.5 ±2.1 mL/minutes/kg, iV=34) umbilical flow. If thromboxane A2 and prostacyclin regulate the umbilical circulation in the human, they exert this effect locally in the fetoplacental unit, and the changes are not reflected by the levels of their metabolites in the maternal peripheral circulation.