The purpose of the study is to determine whether a daily dose of 200 mg progesterone administered vaginally from 20-23 to 34 weeks' gestation will reduce the rate of preterm delivery amongst twin pregnancies.

Multiple gestations account for 2% of all pregnancies, but cause more than 10% of the cases of preterm delivery. Due to increasing maternal age and use of assisted reproduction the number of multiple gestations is larger than ever. If markers of preterm delivery can be identified and means of preventing preterm delivery are found, it will be possible to reduce the number of children admitted to hospital because of prematurity.

Recently, two larger randomised studies investigated the effect of progesterone in singleton pregnancies of women who had previously delivered preterm. They both found a significant reduction in the preterm delivery rate in the progesterone group compared to a placebo group.

The study will be performed as a randomized, double-blind placebo controlled study of twin pregnancies in Denmark and Austria. The women will be randomised in a 1:1 ratio to two groups (progesterone versus placebo). Randomisation will be stratified per centre and according to chorionicity, as the risk of preterm delivery is doubled in monochorionic twins compared to dichorionic twins.

Inclusion Criteria:

• Twin pregnancy
• Informed consent
• 18-23 weeks' gestation
• Participants must be fluent in the language spoken in the respective centres

Exclusion Criteria:

• Age < 18 years
• Known allergy to progesterone or peanuts
• Active thromboembolic disorders or a history of hormone-associated thromboembolic disorders
• Rupture of membranes at the time of inclusion
• Monoamniotic twins
• Pregnancies treated for or with signs of twin-to-twin transfusion syndrome at inclusion
• Multiple pregnancies reduced to twin pregnancies
• Known significant structural or chromosomal fetal abnormality
• Chorionicity not assessed before 15 weeks
• Known or suspected malignancy in genitals or breasts
• Known liver disease