BLOOD AMMONIA CONCENTRATION IN CORD BLOOD DURING PREGNANCY

In vivo studies in several animal species and in vitro studies of human placenta suggest that ammonia is produced within the uteroplacenta and released into the fetal and maternal circulations. Furthermore, the fetal blood ammonia levels in sheep have been found to be significantly greater than maternal levels and inversely correlated with gestational age. Our present study had two main goals: first, to assess whether ammonia may be produced in the human placenta and taken up by the fetus and second, to determine if the fetal blood ammonia levels are greater than maternal and inversely correlated to gestational age. We measured the blood concentrations of ammonia by obtaining both umbilical venous (UV) and arterial (UA) samples obtained from doubly clamped sections of umbilical cord at the time of delivery. Blood gases and glucose were also measured on each sample. Samples were obtained at 96 deliveries (70 vaginal, 26 cesarean section, 18/26 cesarean deliveries not in labor). The deliveries were separated into two groups. Group A included all deliveries, both vaginal and cesarean section, while Group B consisted of only cesarean section prior to onset of labor. The mean gestational age for all deliveries was 38.4 weeks +/- 2.95 (S.D.) with a range of 25-43 weeks of gestation. There was no significant relationship between the fetal ammonia levels and gestational age in either group. In both groups A and B, the UA ammonia concentrations were significantly higher than the paired UV ammonia levels (UV-UA: Group A: -10.00 +/- 17.6, Group B: -9.3 +/- 12.6). Conclusion: These data show no correlation between fetal ammonia levels and gestational age. However, umbilical ammonia concentrations are approximately twice normal adult levels. A negative UV-UA ammonia difference in the fetal circulation disputes uteroplacental production of ammonia, although this finding may reflect the stress of delivery and not steady state in utero conditions. Alternatively, the human fetus may behave differently from the sheep fetus.