To determine whether therapy with dydrogesterone in threatened abortion during the first trimester of pregnancy will improve pregnancy outcome.Prospective open study.Pregnant women presenting to the obstetric and gynaecology clinic admitting center with vaginal bleeding before 13 weeks gestation were evaluated for entry into the study. Women were excluded if they had a history of recurrent miscarriage.Eligible subjects were randomized to receive either dydrogesterone 40 mg stat dose followed by 10 mg twice a day for one week or conservative therapy.One hundred and 54 women were recruited. There was no statistically significant differences between the two groups with regard to pre-treatment status. The continuing pregnancy success rate was significantly (p=0.037) higher in women treated with dydrogesterone (95.9%) compared with women who received conservative treatment (86.3%). The odds ratio of the success rate between dydrogesterone treatment and non-treatment was 3.773 (95% confidence interval: 1.009-14.108).Corpus luteal support with dydrogesterone has been shown to reduce the incidence of pregnancy loss in threatened abortion during the first trimester in women without a history of recurrent abortion.