A Cochrane review included 7 studies with a total of 538 subjects. The use of progestational agent compared with placebo showed a trend towards a reduction of preterm deliveries at less than 37 weeks of gestation (RR 0.62, 95 % CI 0.39 to 0.98; 4 trials, n=293] and an increase in birthweight. The power of the meta-analysis was also limited by the varying types, dosages and routes of administration of progesterone.
Comment: The quality of evidence is downgraded by study quality (inadequate or unclear allocation concealment), by imprecise results (limited study size for each comparison), and by potential reporting bias (only few small trials reported).