Many variables differ between twins and singleton infants

EDITOR—Williams and Poulton report that their 22 adolescent twins had lower blood pressure than singletons.1 They interpret their data as being contrary to the fetal origins hypothesis because they presume that twins, being small at birth, would tend to have higher rather than lower blood pressure in later life. As twins have different patterns of fetal growth from singletons, however, they were specifically excluded from the fetal origins hypothesis.2

There are several reasons why the low birth weight of twins may not have the same significance as intrauterine growth retardation in singleton births. Ultrasound evidence suggests that twins down regulate their growth rate early in gestation, possibly during the first trimester.3 Studies in fetal lambs suggest that early down regulation of fetal growth protects against growth retardation induced by undernutrition in later gestation.4 Finally, the metabolic and endocrine changes associated with growth retardation in singleton infants, including hypoinsulinaemia, are not observed in twins.5

Patterns of growth retardation differ in twins and singletons

EDITOR—Williams and Poulton1 and an accompanying editorial by Susser and Levin2 challenged the fetal origins hypothesis with some vigour. Both articles emphasise the finding that twins had lower blood pressure than singletons. They argue that this provides crucial evidence …