Maternal nutrition and perinatal survival.

The simple relationship between maternal macronutrient status and perinatal survival (increased macronutrient intake --> increased maternal weight and/or weight gain --> increased fetal growth --> improved survival) that is usually posited is no longer defensible. First, maternal weight and weight gain are remarkably resistant to either dietary advice or supplementation; further, increased birth weight attributable to maternal nutrition does not necessarily increase perinatal survival (because prepregnant weight is positively associated with both birth weight and higher perinatal mortality). Finally, whereas dietary supplements during pregnancy may have a modest effect on birth weight in nonfamine conditions (by contrast with a large effect in famine or near-famine conditions), their impact is not mediated by maternal energy deposition. Rather, the component of maternal weight gain associated with accelerated fetal growth is maternal water (presumably plasma) volume.