The venous drainage in the human uterus: respiratory gas studies in normal and fetal growth retarded pregnancies

OBJECTIVE: To determine respiratory gas relationships between the uterine veins and umbilical vein in normal and pregnancies complicated by intrauterine growth retardation. STUDY DESIGN: Respiratory gases were measured in both uterine veins and the umbilical vein in eight normal and 13 pregnancies with intrauterine growth retardation. RESULTS: No significant differences were found in the placental versus nonplacental uterine veins. There was a significant correlation for umbilical and uterine venous values of Po2 (p < 0.002) and Pco2 (p < 0.004) in appropriate-for-gestational-age pregnancies, umbilical venous Po2 was always less than uterine venous Po2, and Pco2 always greater than uterine. The transplacental gradient was significantly higher in intrauterine growth retarded than appropriate-for-gestational-age pregnancies for both Po2 and Pco2. There was a lower uterine oxygen extraction in intrauterine growth retarded pregnancies (p < 0.05). CONCLUSION: There is no consistent relationship between placental venous drainage in each uterine vein and placental location. The human placenta simulates a relatively inefficient venous equilibrator and the larger transplacental gradients in intrauterine growth retarded pregnancies may reflect differences in both perfusion pattern and placental structure.