According to the fetal insulin hypothesis, shared genetic factors lead to suboptimal prenatal growth and to insulin resistance in the parent's later life.1 If this is true, we expect non-insulin dependent diabetes in parents to be associated with lower birth weight among their offspring. Some evidence supports an inverse association between birth weight of offspring and paternal diabetes. 2 3 The association between fetal macrosomia and a mother's diabetes during pregnancy has been established,4 however this is likely to be caused by acute metabolic effects. We used data from the large cohort study of British births in 1958 to evaluate whether a father's non-insulin dependent diabetes or a mother's diabetes starting after childbirth is associated with the birth weight of their offspring.