Some developments in opinions about the placenta as a barrier to oxygen.

The subject of fetal oxygenation has been of interest since the days before Christ. The importance of breathing to adults is only too obvious. The precise nature of the substance breathed was not determined until the discovery of oxygen in the late 18th century. In spite of this it is only normal that speculation should have occurred as to how the fetus in utero in fact received whatever the substance was which adults breathed and which was obviously necessary for survival. Aristotle and Hippocrates assumed that since without breathing adults would die, presumably the fetus also breathed. It was not really until the 16th century that serious debates began about the precise nature of the oxygen supply to the unborn fetus. It will be the purpose of this essay to review in brief some major historical mileposts in the ongoing debates regarding the precise nature of fetal oxygenation. Four major theses, arbitrarily chosen, will be discussed, since they may be said to represent rather crucial ones in the view of obstetricians. The first thesis essentially revolves around the proposition that the maternal and fetal vessels in the placenta are continuous and that there is no need for the fetus to breathe since he would receive all of the necessary vital spirit directly from his mother. This was the question debated from the early 16th century to as late as the end of the 18th century. Once it had been answered to the satisfaction of all with the proof that the maternal and fetal circulations were not continuous, there arose a set of new questions that take us into the more modern era. Here three theses may be put as follows: 1. The placenta is a substantial barrier to the passage of oxygen. 2. The degree of barrier action to oxygen increases with advancing gestation. 3. The barrier to oxygen is a hazard to the fetus. For each of these theses we shall cite the evidence that could be brought forward in favor of or in opposition to it. Even though the individual