Etiology of individual di fferences in bi r th weight

Bi r th weight is in large extent influenced by gestational age. In addi tion genetic and envi ronmental factors determine intrauter ine growth and bi r th weight. The contr ibutions of these factors may be influenced by maternal smoking dur ing pregnancy. We examined bi r th weight and maternal smoking in a sample of 2930 twin pai rs from the Nether lands Twin Register using structural equation model l ing. Gestational age accounted for 27–44% of the var iance in bi r th weight. A lower var iabi l i ty of bi r th weight and a lower association of bi r th weight wi th gestational age was found in twins whose mothers smoked dur ing pregnancy. The var iance not associated wi th gestational age was independent of maternal smoking dur ing pregnancy. A systematic smal ler par t of the var iabi l i ty in bi r th weight was associated wi th var iabi l i ty in gestational age in second born twins compared to first born twins. The her i tabi l i ty of inter individual di fferences in bi r th weight was modest (10% for tw ins wi th non-smoking mothers and 11% for tw ins wi th smoking mothers). Common envi ronmental influences other than gestational age accounted for a sl ightly larger par t of the var iance not associated wi th gestational age (17–20%).