Our aims were: 1. To investigate if women with PCOS who become pregnant using gonadotrophins have a higher incidence of gestational diabetes mellitus (GDM) compared to spontaneously pregnant matched control women, 2. To compare the prevalence of GDM in PCOS women with that in women with hypo/eugonadotrophic hypogonadism and in unexplained infertility and 3. To investigate differences in pregnancy outcomes between the groups. This was a retrospective case-control study. Women with PCOS were matched with a control by age, BMI, and ethnicity. There were 60 women with PCOS, 11 with hypogonadotrophic hypogonadism, 6 with eugonadotrophic hypogonadism, and 12 with unexplained infertility. Control women were those who attended a major public hospital for antenatal care and delivery We found no difference in the prevalence of GDM between the PCOS (22%) and the controls (17%) or between the PCOS and other groups. Women with GDM (diet or insulin controlled) had a significantly higher BMI than women without GDM (p = 0.019). There was no difference in pregnancy outcomes between the groups. There was a significant dependence of babies' birthweight on mother's BMI (p<0.001).