Twin-singleton differences

Twins have a higher occurrence of prematurity, low birth weight, congenital malformations, neonatal morbidity, and perinatal death compared to singletons. However, data spanning back more than a century document that twin-singleton differences generally vanish with age and that, for more recent birth cohorts, the differences are few and generally small after childhood. Even in traits for which there is a mean difference between twins and singletons, it seems plausible in most cases that it is the same factors in twins and singletons that cause variation around their respective mean values. Therefore, twin-singleton differences will rarely be a challenge for using twin research together with other study designs and populations to understand the trait under study.