This is a single-blind randomized controlled trial, aiming to evaluate the effects of vaginal seeding on body mass index as well as allergy risk for cesarean-delivered infants. It will be conducted in Liuyang city of China, and the targeted sample size is 106. All the eligible pregnant women will be randomly assigned to either the intervention or control group, and their babies of the participants will be followed up to 24 months of age.

Many studies have suggested that caesarean-delivered children are at higher risks of developing metabolic and allergic diseases like obesity and asthma, possibly because newborns born by caesarean section were lack of exposure to maternal vaginal flora. It is needed to explore a simple, convenient, and safe intervention strategy to reduce caesarean-related risks. In a recent non-randomized study, the authors found that exposure of caesarean-delivered newborns to maternal vaginal fluid at birth (i.e., vaginal seeding) could partially restore the microbiota of them at 30 days after births, but the long-term health consequences of restoring the microbiota of caesarean-delivered infants remain unclear.

In this randomized study, the investigators aim to examine whether the changes in newborns' microbiota will persist to 24 month of age, and whether vaginal seeding will have any effects on body mass index and allergy risk from birth to 24 months of age. the investigators will enroll a total of 106 pregnant women, and the participants' infants will be followed up at 0 (before hospital discharge), 6, 12, 18 and 24 months. At each follow-up visit, a questionnaire survey including information on feeding, medication, and allergic status will be conducted, infants' height and weight will be measured, and feces will be collected. At 18-month-old visit, infants' venous blood will be also collected for the assay of multiallergen. The primary outcomes were body mass index and allergy risk index. The secondary outcomes included the microbiota profile, allergic symptoms and diseases, overweight/obesity, and adverse effects.

Inclusion Criteria:

• local resident in Liuyang city
• Singleton, term pregnancy (≥37 weeks of gestation)
• Cesarean section before labor starts without maternal complications, or cesarean section after the start of labor but the cervix is less than 3 cm
• Vaginal pH< 4.5 at enrollment

Exclusion Criteria:

• Positive testing for HIV, HBV, syphilis or GBS infection at gestation
• Vaginal infections such as genital herpetic lesions or chlamydia
• Bacterial vaginosis
• Trichomonas or fungous in leucorrhea
• Pregnant women or her spouse with severe allergic diseases, such as asthma and severe drug allergy
• Vaginal pH ≥4.5 at 1-2 hours before the cesarean section
• Other conditions not suitable for intervention as judged by obstetricians

The IPD will be shared upon the request of other researchers, and all the key variables that will be reported in the main paper are planned to be shared

After the publication of the main paper (hopefully before the end of 2022), the data will be available to share to other researchers

Researchers should request the data via the following Email: lihongtian@pku.edu.cn or liujm@pku.edu.cn