Progesterone is an important hormone for the establishment and maintenance of pregnancy.  Because of this important role, progesterone deficiency has been studied since the 1950s as a possible cause of some miscarriages.  Many of the early studies done on progesterone were small and poor quality, making it difficult for doctors to make evidence-based recommendations for its use.   However, more recently there have been two large, high-quality placebo controlled trials (the PROMISE trial and PRISM trial) that have shown encouraging results. 

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