Threatened miscarriage occurs in 10% to 15% of all pregnancies. Vaginal spotting or bleeding during early gestation is common, with nearly half of those pregnancies resulting in pregnancy loss. To date, there is no effective preventive treatment for threatened miscarriage. Chinese herbal medicines have been widely used in Asian countries for centuries and have become a popular alternative to Western medicines in recent years. Many studies claim to show that they can prevent miscarriage. However, there has been no systematic evaluation of the effectiveness of Chinese herbal medicines for threatened miscarriage.