GNRH1 and spontaneous abortion: The DNG + short-acting GnRH-a and ultra-long GnRH-a groups (RR: 0.178, 95%CI: 0.009, 3.433, P = 0.253), the DNG and long GnRH-a groups (RR: 1.455, 95%CI: 0.512, 4.134, P = 0.482), as well as the DNG and dydrogesterone groups (RR: 1.639, 95%CI: 0.873, 3.078, P = 0.124) were identified to have comparable miscarriage rates, respectively.