ObjectiveTo investigate the prevalence of, and associations between, prenatal and perinatal risk factors and developmental vulnerability in twins at age 5.DesignRetrospective cohort study using bivariate and multivariable logistic regression.SettingWestern Australia (WA), 2002–2015.Participants828 twin pairs born in WA with an Australian Early Development Census (AEDC) record from 2009, 2012 or 2015.Main outcome measuresThe AEDC is a national measure of child development across five domains. Children with scores <10th percentile were classified as developmentally vulnerable on, one or more domains (DV1), or two or more domains (DV2).ResultsIn this population, 26.0% twins were classified as DV1 and 13.5% as DV2. In the multivariable model, risk factors for DV1 were maternal age <25 years (adjusted OR (aOR): 7.06, 95% CI: 2.29 to 21.76), child speaking a language other than English at home (aOR: 6.45, 95% CI: 2.17 to 19.17), male child (aOR: 5.08, 95% CI: 2.89 to 8.92), age younger than the reference category for the study sample (≥5 years 1 month to <5 years 10 months) at time of AEDC completion (aOR: 3.34, 95% CI: 1.55 to 7.22) and having a proportion of optimal birth weight (POBW) <15th percentile of the study sample (aOR: 2.06, 95% CI 1.07 to 3.98). Risk factors for DV2 were male child (aOR: 7.87, 95% CI: 3.45 to 17.97), maternal age <25 (aOR: 5.60, 95% CI: 1.30 to 24.10), age younger than the reference category (aOR: 5.36, 95% CI: 1.94 to 14.82), child speaking a language other than English at home (aOR: 4.65, 95% CI: 1.14 to 19.03), mother’s marital status as not married at the time of twins’ birth (aOR: 4.59, 95% CI: 1.13 to 18.55), maternal occupation status in the lowest quintile (aOR: 3.30, 95% CI: 1.11 to 9.81) and a POBW <15th percentile (aOR: 3.11, 95% CI: 1.26 to 7.64).ConclusionBoth biological and sociodemographic risk factors are associated with developmental vulnerability in twins at 5 years of age.