The aim of this study was to evaluate the effect of vaginal natural progesterone on the prevention of preterm birth in IVF/intracytoplasmic sperm injection (ICSI) pregnancies. A single-centre prospective placebo-controlled randomized study was performed. A total of 313 IVF/ICSI pregnant patients were randomized into two groups for either treatment with daily 400 mg vaginal natural progesterone or placebo, starting from mid-trimester up to 37 weeks or delivery. Amongst the patients, there were 215 singleton and 91 twin pregnancies. There was no significant difference in risk of preterm birth among all patients (OR 0.672, 95% CI 0.42-1.0. There was a significantly lower preterm birth rate in singleton pregnancies in the natural progesterone arm (OR 0.53, 95% CI 0.28-0.97) and no significant difference between both arms in twin pregnancies (OR 0.735, 95% CI 0.36-2). In conclusion, the administration of 400 mg vaginal natural progesterone from mid trimester reduced the incidence of preterm birth in singleton, but not in twin, IVF/ICSI pregnancies.
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