High oxygen affinity of maternal blood reduces fetal weight in rats.

In order to obtain information on the physiological significance of the difference in oxygen affinity between maternal and fetal blood, we have examined the effect of an acute increase of the oxygen affinity of maternal blood in pregnant rats, on fetal body weight, fetal brain and liver weights, placental weight, and the hematocrit of fetal blood. The increase in oxygen affinity was produced by exchange-transfusing pregnant rats on day 19 of gestation with blood that had been treated previously with sodium cyanate. As a result of the exchange transfusion, the difference in oxygen affinity between maternal and fetal blood essentially disappeared. Pregnant rats exchanged with normal blood served as controls. On day 21 of gestation, the fetal body weight and the fetal liver weight were significantly smaller by 18% and 25% respectively, in the group where the oxygen affinity of the maternal blood was acutely raised when compared to the controls. Also, the hematocrit of the fetal blood was significantly higher in the group where mothers had the high blood oxygen affinity. Placental weight and fetal brain weight were not significantly altered. We infer, that the reduction of fetal weight is due to fetal hypoxia which is caused by the abolishment of the difference in oxygen affinity between maternal and fetal blood. Human infants, whose birth weight is too small for their gestational age have a significantly increased postnatal mortality compared to babies with normal weight. We suppose that the increased mortality of newborns who are too small for their gestational age, represents a selective pressure that has favoured the evolution of a high oxygen affinity of fetal in comparison with maternal blood.