Unusual maternal–fetal blood glucose concentrations in Weddell seal

FETAL whole blood glucose levels are usually lower than, and fluctuate with, maternal levels. That is, a favourable concentration gradient is maintained to allow for the facilitated transfer of glucose across the placenta from mother to fetus1,2. This situation is apparently reversed in the Weddell seal. The concentration of glucose in maternal whole blood is lower than that in fetal blood during at least three metabolic states: in the resting–fasting state, in simulated diving, and in recovery from diving3. We report here that one factor contributing to this unusual relationship between maternal and fetal blood glucose is the relatively low gradient between normal blood levels in fetus and mother. This is made possible by high haematocrit values and a low turnover of glucose in both parent and fetus.