Administration of vaginal progesterone
For the study participants, administration of vaginal progesterone was started between 16+0 and 19+6 weeks of gestation. We used 200 mg vaginal progesterone capsules (UTROGESTAN® vaginal capsules 200 mg, Fuji Pharma Co., Ltd., Japan). The women self-administered the tablets vaginally once daily at night. Progesterone administration was discontinued at 34 weeks of gestation or at the time of delivery, rupture of membranes, or massive uterine bleeding, whichever occurred first. At approximately 26 weeks of gestation, study participants were examined for side effects of progesterone through the following blood tests: (1) liver function: aspartate aminotransferase [AST] and alanine aminotransferase [ALT] and (2) coagulation function: D-dimer, and further tests were conducted if the test results were abnormal.