OBJECTIVE Alpha Lipoic Acid (ALA) is a safe natural molecule that exerts a selective immunomodulating activity with antioxidant and anti-inflammatory properties. This randomized controlled clinical trial (RCT) tested the effect of the vaginal administration with ALA or Progesterone, in subchorionic hematoma resorption in women with threatened miscarriage. PATIENTS AND METHODS 400 mg of vaginal Progesterone or 10 mg of vaginal ALA were administered to sixty-two pregnant women, in the first trimester of gestation with threatened miscarriage and subchorionic hematoma. Controls were patients who chose not to receive any treatment. RESULTS In the ALA group the subchorionic hematoma was reabsorbed more quickly in comparison with the progression detected in Progesterone group (p ≤ 0.05). The other parameters checked (pelvic pain and vaginal bleeding) did not show any significant difference and a smaller number of miscarriages was recorded in the ALA group, compared to Progesterone group. CONCLUSIONS Our data provides the first evidence of the efficacy of ALA, administered by vaginal route, in the healing process of patients with threatened miscarriage, thus supporting the normal course of pregnancy. CLINICAL TRIAL REGISTRATION NUMBER NCT02601898 (ClinicalTrials.gov registry).