Fetal oxygen uptake in normal and IUGR pregnancies

Intrauterine growth restriction (IUGR) is characterized by the failure of the fetus to grow at a normal rate in utero. The role of placental function and the relationship between maternal and fetal circulation is crucial for efficient exchanges of oxygen and nutrients. Umbilical oxygen uptake (O2 umb uptake) has been estimated in human pregnancies only in acute experiments at the time of caesarean section. Since umbilical blood flow can be obtained by ultrasound in utero, we studied normal and IUGR pregnancies in order to evaluate fetal oxygen uptake.Methods. 18 normal and 36 IUGR pregnancies were studied at the time of elective caesarean section (gestational age: 38.9±1.6 and 32.0±2.5 wks). Average fetal and placental weights were 3217±314 and 472±82 g in normals, 1287±461 and 254±174 g in IUGR. Umbilical vein volume flow (Qumb) was measured by ultrasound before caesarean section and blood samples from umbilical vein (uv) and artery (ua) were obtained. Blood gases and acid-base balance were evaluated. Umbilical oxygen uptake was calculated as O2 umb uptake = Qumb* (uv-ua) O2 content. Results. IUGR pregnancies showed a significant reduction in Qumb (98.5±6.6 vs 234.8±12.1 mL/min; p<0.01) but similar Qumb/Kg of fetal weight compared to normals (71.8±31.3 vs 72.5±17.1 mL/min/Kg). (uv-ua) O2 content, and O2 umb uptake/Kg were significantly reduced in IUGR (1.79±0.72 vs 3.3±0.24 mmol/L; 0.12±0.01 vs 0.24±0.01 mmol/min/Kg; p<0.01). Conclusions. Oxygen uptake per Kg obtained in normal pregnancies was surprisingly similar to the values reported in chronically catheterized animals. However, IUGR fetuses showed a significant reduction in both blood and oxygen supply: this latter was reduced of » 50% on a per kg basis.