Physiological development of a mixed metabolic and respiratory umbilical cord blood acidemia with advancing gestational age.

OBJECTIVE To investigate respiratory and metabolic components of gestational age-dependent umbilical cord blood gas changes. STUDY DESIGN Cord blood gases were determined in 1336 vigorous singletons with uncomplicated cephalic vaginal delivery at 37-43 weeks. Linear regression analysis and non-parametric statistics were used with a P < 0.05 being significant. RESULTS Gestational age correlated negatively with arterial pH and HCO3- and positively with pCO2 and base deficit. Venous blood showed corresponding changes except for pCO2, which was independent of gestational age. Arterial pCO2, but not venous, correlated positively with birthweight deviation from the mean. The fractional fetal CO2 production per birthweight unit and the fractional placental CO2 clearance per placental weight unit were not correlated with gestational age. CONCLUSIONS A mixed respiratory and metabolic cord blood acidemia develops with advancing gestational age. The respiratory component depends on an increased 'CO2 load' from the growing fetus and not on deterioration of placental gas exchange. The etiology of the metabolic component is yet unknown.