Epidemiological evidence shows that women living in affluent circumstances have bigger babies with a lower mortality than underpriviliged women. How much of that effect is due to nutrition alone is not known but supplementary feeding in pregnancy of chronically ill nourished women does appear to increase mean birth weight, and famine conditions in a basically well nourished community reduce the birth weight; in both cases the birth weight difference is relatively small and could be accounted for by no more than fetal adipose tissue.
The fetus may be much less vulnerable to vagaries of maternal diet than has been thought because of protective physiological mechanisms associated with pregnancy. Firstly the mother's energy balance changes, so that if she has access to extra food in the first half of pregnancy she will store large amounts of depot fat as an energy buffer against possible privation later. Secondly, there are widespread changes in nutrient metabolism one of which is to lower plasma levels of most nutrients, and that may tip the balance of advantage away from maternal tissue towards the placenta. Finally, the placenta itself has developed elaborate mechanisms to acquire nutrients from the maternal circulation. A notable exception to that rule is glucose for which no active transport mechanism has evolved and which might therefore be regarded as a low priority nutrient; it may be that the generous supply of glucose for the fetus which would be provided by a well fed woman does little more than allow the fetus to build up its fat stores.
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