Renewed interest in nutrition during pregnancy has been generated by the hypothesis that adult disease has origins in early life. Animal experiments clearly show that altering maternal diet before and during pregnancy can induce permanent changes in the offspring's birth size, adult health and lifespan. Among women living in Western societies, cigarette smoking is the most important factor known to reduce fetal growth, followed by low pre-pregnancy weight and low gestational weight gain. Obesity is also associated with pregnancy complications and adverse neonatal outcomes, so inadequate or excessive energy intake is not optimal for the developing fetus. Against a history of inconsistent results, several recent studies suggest that in Western settings the balance of macronutrients in a woman's diet can influence newborn size. Effects appear to be modest, but this relationship may not encapsulate the full significance for health of the child, as there is emerging evidence of associations with long-term metabolic functioning that are independent of birth size. Consequences of inadequate maternal nutrition, for the offspring, may depend on timing during gestation, reflecting critical windows for fetal development. Where women are not malnourished, changing a woman's nutritional plane during pregnancy may be detrimental to the unborn baby, and systematic reviews of the literature on dietary supplementation during pregnancy indicate few benefits and possible risks. In view of this, improved diet before pregnancy deserves greater attention.