BACKGROUND: Progesterone, a female sex hormone, is known to induce secretory changes in the lining of the uterus essential for successful implantation of a fertilised egg. It has been suggested that a causative factor in many cases of miscarriage may be inadequate secretion of progesterone. Therefore, progestogens have been used, beginning in the first trimester of pregnancy, in an attempt to prevent spontaneous miscarriage. OBJECTIVES: To determine the efficacy and safety of progestogens as a preventative therapy against miscarriage. SEARCH STRATEGY: We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (January 2008), CENTRAL (The Cochrane Library 2006, Issue 4), MEDLINE (1966 to June 2006), EMBASE (1980 to June 2006), CINAHL (1982 to June 2006), NHMRC Clinical Trials Register (June 2006) and Meta-Register (June 2006). We searched references from relevant articles, attempting to contact authors where necessary, and contacted experts in the field for unpublished works. SELECTION CRITERIA: Randomised or quasi-randomized controlled trials comparing progestogens with placebo or no treatment given in an effort to prevent miscarriage. DATA COLLECTION AND ANALYSIS: Two review authors assessed trial quality and extracted data. MAIN RESULTS: Fifteen trials (2118 women) are included. The meta-analysis of all women, regardless of gravidity and number of previous miscarriages, showed no statistically significant difference in the risk of miscarriage between progestogen and placebo or no treatment groups (Peto odds ratio (Peto OR) 0.98; 95% confidence interval (CI) 0.78 to 1.24) and no statistically significant difference in the incidence of adverse effect in either mother or baby. In a subgroup analysis of three trials involving women who had recurrent miscarriages (three or more consecutive miscarriages), progestogen treatment showed a statistically significant decrease in miscarriage rate compared to placebo or no treatment (Peto OR 0.38; 95% CI 0.20 to 0.70). No statistically significant differences were found between the route of administration of progestogen (oral, intramuscular, vaginal) versus placebo or no treatment. AUTHORS' CONCLUSIONS: There is no evidence to support the routine use of progestogen to prevent miscarriage in early to mid-pregnancy. However, there seems to be evidence of benefit in women with a history of recurrent miscarriage. Treatment for these women may be warranted given the reduced rates of miscarriage in the treatment group and the finding of no statistically significant difference between treatment and control groups in rates of adverse effects suffered by either mother or baby in the available evidence. Larger trials are currently underway to inform treatment for this group of women.