Gestational age impacts birth to placental weight ratio and umbilical cord oxygen values with implications for the fetal oxygen margin of safety

Background: We determined the impact of gestational age (GA) from near term to term to post-term on birth/ placental weight ratio and cord oxygen values with implications for placental transport efficiency for oxygen, fetal O-2 consumption relative to delivery or fractional O-2 extraction, and oxygen margin of safety. Materials and methods: A hospital database was used to obtain birth/placental weight ratios, cord PO2 and other information on patients delivering between Jan 1, 1990 and Jun 15, 2011 with GA > 34 completed weeks (N = 69,852). Oxygen saturation was calculated from the cord PO2 and pH data, while fractional O-2 extraction was calculated from the oxygen saturation data. The effect of GA grouping on birth/placental weight ratio, cord PO2, O-2 saturation, and fractional O-2 extraction values, was examined in all patients adjusting for pregnancy and labor/delivery covariates, and in a subset of low-risk patients. Results: Birth/placental weight ratio and umbilical venous O-2 values increased with advancing GA, supporting the conjecture of increasing placental transport efficiency for oxygen. However, umbilical arterial O-2 values decreased while fractional O-2 extraction increased with successive GA groupings, indicating that fetal O-2 consumption must be increasing relative to delivery. Conclusions: Fetal O-2 consumption can be seen as ever 'outgrowing' O-2 delivery over the last weeks of pregnancy and leading to a continued lowering in systemic oxygen levels. While this lowering in oxygen may trigger feedback mechanisms with survival benefit, the 'oxygen margin of safety' will also be lowered increasing perinatal morbidity and mortality which appear to be hypoxia related.