The aim of the study is to examine the effect of exposure of cesarean section (CS) delivered neonates to a natural condition of birth, i.e. to the microbiota of the birth canal, on the acquisition of microbial genes during development of the microbiome at multiple body sites, immune system maturation and allergy risk in childhood

The primary outcome is to examine if exposure to the maternal vaginal and fecal microbiota directly after birth will halfen the cumulative incidence of Immunoglubulin E (IgE)-associated allergic disease at 2 years of age in CS delivered infants, compared with non-treated CS-delivered infants.

The secondary outcomes are to compare the community structure of microbes from mothers and their infants and immunological programming of infants delivered via scheduled CS, with or without exposure to the maternal vaginal and fecal microbiota, from birth until two years of age. The primary and secondary outcomes will also be compared with a reference group of vaginally delivered infants. Sex differences in the incidence and prevalence of allergic diseases have been described. The anticipated benefit of the intervention can be implemented in clinical practice regardless of sex, so that improved conditions for good health are created.

Inclusion Criteria:

• Infants of healthy mothers with uncomplicated pregnancies at term, mothers of any ethnic or social background that can speak, read, and understand Swedish to the extent that they can consent in Swedish. An additional inclusion criterion for mothers in the CS groups is vaginal pH ≤ 4 at the time of birth.

Exclusion Criteria:

• Infants of mothers with pre-eclampsia or complicated pregnancies, mothers <18 years or >40 years of age, genital herpes simplex, genital warts, HIV, Hepatitis B or complications during delivery, infants delivered prior to 37 weeks of gestation. Additional exclusion criteria for mothers in the CS group: Mothers positive to Group B Streptococcus, Group A Streptococcus, bacterial vaginosis, vaginal pH > 4 at the time of birth. Mothers with fecal samples positive for Salmonella, Shigella, Campylobacter or Yersinia.

Samples will be stored at Karolinska Biobank and metadata will be linked to these samples with access for all researchers in the study. No data will be available for researchers outside the consortium.

Mothers who give birth by CS will be randomized to expose their neonate to samples of their vaginal and fecal microbiome after birth or to placebo.