This population-based study determined the impact of co-twin gender on twin intrauterine growth in addition to their infant gender, maternal height, maternal age and parity on intrauterine growth rate of singletons and twins. All singletons and twins born in Western Australia during the period of 1980 to 1995 were considered for the study. The multiple linear regression models showed that 76% of the variance in the mean birthweight was explained by the selected variables for twins and 51% for singletons. Twins grew more slowly than singletons from 26 weeks gestation. Among twins, opposite-sex twin pairs grew consistently faster than like-sex twins. Primiparous twin pairs grew more slowly than subsequently born twins. These regression equations can be used to assess the appropriateness of intrauterine growth in twin pairs of various gender combinations.