Objective: Our purpose was to examine the regulation of fetal serum concentrations of insulin (C-peptide), insulin-like growth factor-I, insulin-like growth factor-II, and insulin-like growth factor binding protein-1, which are growth-regulating factors in the fetus, in monozygotic and dizygotic twin pairs.
Study design: Cord serum samples were collected from 110 twin pairs and compared with 178 nonsibling singleton pairs with the same gestational age. Five twin pairs were excluded from the statistical analyses because of severe intrauterine growth restriction and placental abnormalities in one. Zygosity was assigned by histologic examination of the placenta and by a questionnaire sent to the mother when the twins were > or = 6 months old. Analyses included the calculation of correlation coefficients, between-pair variation, and univariate genetic analysis.
Results: Cord serum C-peptide concentrations were highly correlated in monozygotic (r = 0.94) and dizygotic twins (r = 0.79) but not in singleton pairs (r = -0.05); the between-pair variation was also smaller in twins than in singletons. Genetic analysis demonstrated a large contribution of the common environment to the variance in C-peptide concentrations (80%) and a smaller genetic contribution (12%). Insulin-like growth factor-I concentrations were better correlated in monozygotic (r = 0.82) than in dizygotic twins (r = 0.42), with a smaller between-pair variation in the former group (22% +/- 4% vs 51% +/- 5%). Univariate genetic analysis indicated that insulin-like growth factor-I levels were regulated predominantly by genetic mechanisms (93% in boys and 77% in girls). The regulation of insulin-like growth factor-II was more complex, with a gender-specific genetic contribution (50% for both sexes combined, 63% for girls but only 5% for boys). Insulin-like growth factor binding protein-1 was regulated by genetic mechanisms (41%) and the common environment (32%) but also by the specific or unique environment of each fetus (27%). In all five twins with intrauterine growth restriction of one member insulin-like growth factor binding protein-1 concentrations were markedly higher in the growth-restricted fetus.
Conclusions: Insulin secretion in twin fetuses is determined primarily by their common, probably maternal, environment, whereas insulin-like growth factor-I production is predominantly genetically regulated. Insulin-like growth factor-II and insulin-like growth factor binding protein-1 are regulated by both genetic and environmental factors. Of these growth-regulating factors, insulin-like growth factor binding protein-1 appears to be the best marker of intrauterine growth restriction in the individual case.