a1 Vrije Universiteit, Amsterdam, The Netherlands. firstname.lastname@example.org
a2 Vrije Universiteit, Amsterdam, The Netherlands.
Birth weight is in large extent influenced by gestational age. In addition genetic and environmental factors determine intrauterine growth and birth weight. The contributions of these factors may be influenced by maternal smoking during pregnancy. We examined birth weight and maternal smoking in a sample of 2930 twin pairs from the Netherlands Twin Register using structural equation modelling. Gestational age accounted for 27–44% of the variance in birth weight. A lower variability of birth weight and a lower association of birth weight with gestational age was found in twins whose mothers smoked during pregnancy. The variance not associated with gestational age was independent of maternal smoking during pregnancy. A systematic smaller part of the variability in birth weight was associated with variability in gestational age in second born twins compared to first born twins. The heritability of interindividual differences in birth weight was modest (10% for twins with non-smoking mothers and 11% for twins with smoking mothers). Common environmental influences other than gestational age accounted for a slightly larger part of the variance not associated with gestational age (17–20%).
(Received March 20 1998)
(Accepted August 25 1998)
c1 Correspondence: Dr GCM van Baal, Dept of Biological Psychology, De Boelelaan 1111, 1081 HV Amsterdam, The Netherlands. Tel: (31) (20) 444 8802; Fax: (31) (20) 444 8832