In a prospective, randomised trial, 100 pregnant women with >/= one prior spontaneous pre-term birth were randomised into two groups. Group 1 women received 100 mg natural micronised progesterone intravaginally once daily from 20-24 weeks' gestation until 36 weeks. Group 2 women did not receive progesterone. Both groups were regularly supervised until delivery. Pre-term birth (<37 and <34 weeks) and other maternal, neonatal outcomes were primary and secondary outcomes, respectively. Chi-square test and Fisher exact test were used to compare categorical variables. Independent sample t-test and one-way ANOVA were used to compare continuous variables and multiple comparisons, respectively. Pre-term births <37 weeks were significantly lower in Group 1 (12% vs 38%, p = 0.002), but pre-term births <34 weeks were similar. The mean birth weight of neonates born to women in Group 1 was significantly higher (2800 vs 2,500 g, p = 0.023). We concluded intravaginal administration of 100 mg of natural micronised progesterone significantly reduced the incidence of pre-term birth <37 weeks in women with > or = one prior pre-term birth. Future research is warranted to assess the long-term safety and efficacy of progesterone.