BACKGROUND AND AIMS: fecundability of humans is quite low compared to the majority of other species and further impairment of fertility has been noticed in recent decades. An estimated twenty percent of pregnancies miscarry. Early and late pregnancy loss leaves great emotional stress and burdens both partners with a lot of unanswered questions. Preterm birth (PTB), defined as a delivery of a live born infant before 37 weeks or 259 days of pregnancy complicates a bit over 12% of all US deliveries and it is one of main causes of neonatal morbidity and mortality. Therefore it is clear that there is a need for further research to discover a successful treatment to these conditions. Since progesterone is the essential pregnancy hormone, various randomized controlled studies have been made with the aim to bring clarity to wether or not the supplementation of progesterone really is effective in improving the pregnancy outcome when used as a treatment for threatened miscarriage and/or preterm delivery. The aim of this paper is to present a comprehensive review of literature on the effects of progesterone supplementation in pregnancy. With ambiguous results of research so far, a better understanding of progesterone’s role in pregnancy is being sought for. ----- METHODS: PubMed, Cochrane Library and Google scholar were primary sources for finding relevant material. Search was limited to English articles and terms included were: progesterone in pregnancy; recurrent miscarriage; recurrent pregnancy loss; preterm delivery; habitual abortions and therapy. The main criteria in the selection process was to find a good selection of published evidence-based research as well as avoiding outdated articles. ----- FINDINGS: There is still inconsistency of results regarding progesterone’s role as therapy or prophylaxis in pregnancy with results from two of the newest and largest studies contradicting each other. It is therefore still questionable whether or not progesterone should still be recommended as therapy or prophylaxis in pregnancy and which of the factors are of the greatest value when estimating the possible outcome of its use. Further research with larger groups is needed.