Progesterone supplementation reduces the risk of spontaneous preterm delivery in women with a previous history of spontaneous preterm delivery in a singleton pregnancy and in women with a short cervix on ultrasound in the current pregnancy. ● For women with a previous history of spontaneous preterm delivery in a singleton pregnancy, we recommend treatment with progesterone. We recommend intramuscular injection of hydroxyprogesterone caproate rather than vaginal progesterone, starting at midtrimester (16-20 gestational weeks) until 36 gestational weeks. We apply a dose of 250 mg per week. We give the same treatment to women with singleton pregnancies who have a previous history of preterm delivery in twin pregnancies. For women with a short midtrimester cervix (defined as ≤20 mm by 24 gestational weeks) without a previous history of spontaneous singleton preterm delivery, we recommend vaginal treatment with progesterone until the 36th gestational week. Other reasonable options include vaginal suppositories (100mg or 200mg), gels (90mg) or tablets (100mg micronized progesterone). Available Angiotensin ● Routine progesterone supplementation does not appear to be helpful in preventing preterm delivery in non-elective multiple pregnancies. However: male offspring exposed to exogenous progesterone are at increased risk of hypospadias; but this risk is limited to those exposed before 11 weeks of gestation, and thus is not relevant to women with a prior history of preterm birth, as they do not receive medication until after 16 weeks of gestation. For women with a previous history of spontaneous preterm labor in twin pregnancies, the authors applied hydroxyprogesterone caproate. In women with a current twin pregnancy with a short cervix, the authors administered progesterone vaginally. In cases of preterm premature rupture of membranes (PPROM) or in cases of preterm labor without delivery, routine progesterone supplementation does not seem to be helpful in preventing preterm labor. There is no information on the efficacy of progesterone supplementation in women with a positive fetal fibronectin test. The role of progesterone supplementation in women who have undergone cervical cerclage is unclear.