Maternal and fetal blood ammonia concentrations in normal term human pregnancies.

Background: The current evidence on the primary source of ammonia production in the human fetoplacental unit is potentially misleading.

Objective: The aim of the present investigation was to determine the concentration of ammonia in human maternal and fetal blood at birth and to compare them with published data in late gestation sheep.

Methods: In 12 normal human pregnancies, umbilical arterial and venous and maternal venous blood was sampled, and whole blood ammonia concentrations were measured. Data from 12 pregnant sheep and fetuses from our previous studies were utilized for comparison.

Results: The human fetus at delivery has higher concentrations of ammonia (60-80 microM) than the late gestation fetal lamb (25-35 microM). In the human, the arterial umbilical ammonia concentration exceeds the venous umbilical concentration, indicating a net ammonia production by fetal tissues. In sheep, the venous umbilical ammonia concentration exceeds the arterial umbilical concentration, indicating the net placental ammonia production.

Conclusions: In contrast to fetal lambs, human fetuses exhibit a net production of ammonia, which may reflect differences in biologic state or a species difference.