 In a double-blind, placebo controlled study performed in our Institution, Fonseca et al., investigated the use of micronized natural progesterone, crafted in vaginal capsules of 100mg, for the prevention of preterm delivery. The study involved 142 singleton pregnancies at high risk for premature birth previous premature birth, cerclage and uterine malformation. The progesterone was introduced from 24 weeks to 34 weeks gestation. The authors observed a significant reduction of premature birth before 37 weeks 28,1 within the placebo group versus 13,8 within the treated group and below 34 weeks 18,6 within the placebo group versus 2,8 within the treated group. Also, it was observed a reduction in the frequency of the uterine contractions in the progesterone group. 