Inclusion Criteria:  Aged 20-40 (inclusive); Body mass Index (BMI) 18.5-27.9kg /m2 (inclusive) Single and twin pregnancy; Premature delivery at 24-36 weeks; No antibiotics, prebiotics or probiotics were used one month before feces collection (such as Bifidobacterium triplex live powder, Lactobacillus acidophilus tablet, compound Lactobacillus acidophilus tablet, Bacillus subtilis diplex live intestinal capsule, etc.); Preeclampsia patients accounted for about 1/10 of each group; Use dexamethasone I.M. 5mg Q12h regimen (or other dexamethasone administration regimen) to promote fetal lung maturation.  Exclusion Criteria:  Ectopic pregnancy; Suffering from diabetes, fetal distress, serious infectious diseases (such as sepsis, septic shock), fever; Those who have taken glucocorticoid drugs within 2 weeks before joining the clinical trial; Those who took clindamycin during the study period; Congenital fetal malformation or fetal hypoxia in early pregnancy; Convulsive patients; HIV/HCV/ HEPATITIS A, drug abuse history; Suffering from chorioamnitis, endometritis; Placental abruption, severe intrauterine bleeding; Pregnant women whose cervical dilation is greater than or equal to 4 cm or whose cervical length is less than or equal to 20 mm by ultrasound examination; Pregnant women who took food or drugs during the study that might affect the safety of the fetus; Pregnant women participating in other clinical trials.