Maternal weight gain, infant birth weight, and diet: causal sequences.

The causal sequence maternal nutrition----maternal weight gain----infant birth weight is not sustained by available evidence except under extreme nutritional deprivation. For maternal weight change, diet effects of near starvation are unequivocal. With chronic undernutrition or social deprivation, diet effects are inapparent or modest (conditional on pregnancy stage, diet supplement, and prepregnancy weight). For birth-weight change, diet effects of near starvation are likewise unequivocal and modest with chronic undernutrition or social deprivation. The complete causal sequence has been demonstrated only below a famine threshold. Outside famine, effects are modest (conditional on baseline nutrition, timing, and content of diets, possibly also on infant sex and energy expenditure). High-protein concentrations have produced adverse effects. Micronutrients and consequent fluid retention could have favorable effects. Diet effects on birth weight apparently bypass maternal weight change. Hence, to enhance birth weight, maternal diet appears to deserve more attention than does weight gain.