Prolonged Progesterone to Prevent Preterm Birth From IVF - ET

Inclusion criteria are: age 20-40 years; meeting the indications for assisted reproductive technology (ART) and undergoing fresh embryo transfer with in vitro fertilization (IVF); singleton pregnancy; and willingness to participate and provide written informed consent. Exclusion criteria are: age \>40 years; history of second-trimester miscarriage or cervical insufficiency; ovarian dysfunction or relevant surgical history; uterine malformation; diseases associated with abnormal uterine cavity morphology; abnormal cervical morphology or function, or history of cervical surgery; refractory vaginitis; multiple pregnancy; concomitant severe medical or surgical diseases; participation in another clinical trial within 90 days prior to randomization, or use of an investigational drug within 30 days; inability to complete long-term follow-up or poor compliance; and any other condition that, in the investigator's judgment, may affect trial implementation or outcome evaluation.