Predominance of Cephalic Presentation at Birth: An Oxygen Delivery-Based Concept

Background: Fetuses closer to term present predominantly as cephalic, and by the time of delivery 96-97% of them are born headfirst. To date, no coherent hypothesis has been put forth to explain the phenomenon. Observation: As pregnancy advances, there is a progressive increase in cephalic presentation; by 25 weeks, 60% are cephalic, at 35 weeks 90% cephalic and at term only less than 3-4% remain breech. Hypothesis: Oxygen delivery to the fetus comes from the environment through the parturient and it must traverse a very long passage to get to the fetal tissues making oxygen delivery a very precarious process. The fetus develops several compensatory mechanisms to enhance the delivery of oxygen—higher hemoglobin level, hemoglobin with greater affinity for oxygen, the double Bohr phenomenon to improve the diffusion of oxygen from the parturient and lastly redesign of the fetal circulation to increase flow to the brain whilst in the state of near chronic hypoxia whilst in utero. The better flow of blood to the head and therefore more oxygen to the brain makes this organ the densest causing it to progressively present cephalic as pregnancy advances. Implications: The hypothesis aligns well with not only the observed progressive cephalic phenomenon but also the long-term subtle cognitive deficits and neurological sequelae observed in several studies of those born breech. Conclusion: Cephalic presentation at birth is driven by better blood flow to the brain as another compensatory mechanism to cope with hypoxia.