Abstract :
Keywords Gestational age; Placenta; Fetal oxygen; Fetal growth Highlights * A hospital database determined the impact of advancing gestational age on birth/placental weight and cord oxygen values. * Birth/placental weight and cord vein O.sub.2 increased with GA indicating increasing placental transport efficiency for oxygen. * Cord artery O.sub.2 decreased and O.sub.2 extraction increased with GA indicating increasing fetal O2 consumption to delivery. * Fetal O.sub.2 consumption 'outgrows' O.sub.2 delivery over the last weeks of pregnancy lowering the oxygen margin of safety. Abstract 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.sub.2 consumption relative to delivery or fractional O.sub.2 extraction, and oxygen margin of safety. Materials and methods A hospital database was used to obtain birth/placental weight ratios, cord PO.sub.2 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 PO.sub.2 and pH data, while fractional O.sub.2 extraction was calculated from the oxygen saturation data. The effect of GA grouping on birth/placental weight ratio, cord PO.sub.2, O.sub.2 saturation, and fractional O.sub.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.sub.2 values increased with advancing GA, supporting the conjecture of increasing placental transport efficiency for oxygen. However, umbilical arterial O.sub.2 values decreased while fractional O.sub.2 extraction increased with successive GA groupings, indicating that fetal O.sub.2 consumption must be increasing relative to delivery. Conclusions Fetal O.sub.2 consumption can be seen as ever 'outgrowing' O.sub.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. Author Affiliation: (a) Department of Obstetrics and Gynecology, University of Western Ontario, London, Canada (b) Department of Physiology and Pharmacology, University of Western Ontario, London, Canada (c) Department of Pediatrics, University of Western Ontario, London, Canada (d) Lawson Health Research Institute, University of Western Ontario, London, Canada (e) Interdisciplinary Centre for Health & Society, University of Toronto, Toronto, Canada * Corresponding author at: Department of Obstetrics and Gynecology, London Health Sciences Centre, Victoria Hospital, 800 Commissioners Road E, London, Ontario N6A 5W9, Canada. Article History: Received 14 December 2020; Revised 3 November 2021; Accepted 17 November 2021 Byline: Bryan S. Richardson [brichar1@uwo.ca] (a,b,c,d,*), Barbra de Vrijer (a,d), Hilary K. Brown (e), Larry Stitt (a), Sheryl Choo (a), Timothy R.H. Regnault (a,b,d)