Objectives To estimate the number of pregnant women in New South Wales who had not been tested for gestational diabetes mellitus (GDM) during the period 1991‐1994. Design The number of women not tested for GDM was estimated from the recorded data available in the NSW Midwives Data Collection (MDC) annual reports and compared with three incidence surveys. Main outcome measure The number of pregnant women not tested for GDM. Results Over the four‐year period, the average annual recorded MDC incidence of GDM was 2.2%. This percentage was adjusted up to 3.3% after review of the MDC validation study and an incidence study. However, the expected incidence of GDM from three incidence surveys was 6.6%. Thus, half of the pregnant women in NSW do not appear to have been tested for GDM. Conclusion For the four‐year period 1991‐1994, an estimated 50% of women in NSW were probably not tested for GDM.