Invited commentary: Timing and types of cardiovascular risk factors in relation to offspring birth weight.

Birth weight is associated with later-life cardiovascular risk. A new study by Romundstad et al. (Am J Epidemiol 2007;166:1359-1364) challenges us to consider influences on birth weight with respect to timing and type. Timing of effects on birth weight, according to the "fetal origins hypothesis," is in utero. Alternatively, familial aggregation--genetics or shared environment--may explain birth weight and suggests prepregnancy influences. The Romundstad et al. findings support familial effects: maternal metabolic factors predicted birth weight for gestational age. However, because maternal physiology sets the fetal environment, these data do not necessarily counter the fetal origins hypothesis. Types of maternal metabolic influences demonstrated by Romundstad et al. include elevations in blood pressure being associated with lower birth weight for gestational age, whereas unfavorable glucose and lipid levels were associated with higher birth weight. These findings are consistent with the authors prior hypothesis that vascular dysfunction and metabolic profile (glucose and lipids) have divergent effects during pregnancy. Moreover, these new data underscore that both extremes of birth weight may be related to cardiovascular risk. Few data sets contain prepregnancy, pregnancy, and childhood information. Without all such time points, life course effects will remain only partially understood. It is hoped that studies such as the forthcoming National Children's Study will generate critical understanding of this issue.