Purpose: This study examines the expressive language and speech of twins, relative to singletons, at 3 and 5 years, with the aim of determining if a twinning effect occurs during this developmental period. The possibility of twins outgrowing a twinning effect was investigated. Method: A weighted population-based sample of 185 twins and 1,309 closely spaced singletons who participated in the Growing Up in Ireland study was analyzed. Their development was compared using a standardized expressive vocabulary assessment and parent interview at 3 and 5 years as well as teacher reports at 5 years. Causal steps mediation analyses were subsequently conducted to determine if gestation and birth weight mediated twin-singleton differences in expressive vocabulary development. Results: Twins' expressive vocabulary scores were slightly lower than those of singletons at 3 years, but there was no significant difference at 5 years. More parents of twins reported "a little" concern about their child's expressive language and speech skills at 3 years, although there was no significant twin--singleton difference 2 years later. Teachers' ratings of twins' and singletons' expressive language and speech skills at 5 years were comparable. Shorter gestations and lower birth weights mediated twins' lower expressive vocabulary scores at 3 years. Conclusions: A very small twinning effect on expressive vocabulary development exists at 3 years and is outgrown by 5 years. Twins born after shorter gestations or at a lower birth weight are at an increased risk of having less developed expressive vocabulary skills at 3 years.