Objective The objective of this study was to follow up and evaluate the statewide first-trimester combined screening programme for Down syndrome and trisomy 18 at Genetic Health Services Victoria, Australia.Design Retrospective population cohort.Setting Maternal Serum Screening Laboratory records. Methods Screening results were matched to Victorian perinatal and birth defect data via record linkage, with an ascertainment of 96.8% of pregnancy outcomes. Manual follow up with health professionals increased ascertainment to more than 99%.Main outcome measures Fetal Down syndrome or trisomy 18, and combined screen results, to calculate test characteristics.Results Using a risk threshold of 1 in 300 at time of ultrasound, the sensitivities for standard first-trimester combined screening and augmented 13-week combined screening for Down syndrome were 87.3 and 90.5% and the false-positive rates (FPR) were 4.1 and 3.9%, respectively. The sensitivity for trisomy 18 was 66.7% (10/15, 95% CI 42.8-90.5%) with a 0.4% FPR and 15.2% positive predictive value (1 in 250 risk threshold).Conclusions The combined use of record linkage and manual follow-up techniques was effective in ascertaining more than 99% of pregnancy outcomes for calculations of accurate test characteristics of the combined screen. The sensitivity for Down syndrome at Genetic Health is comparable to similar populations. However, the sensitivity for trisomy 18 is lower than that elsewhere, which may reflect the overall low birth prevalence of trisomy 18 and associated small numbers in this particular cohort.