Stinson LF 1, Payne MS1, Keelan JA1 1Division of Obstetrics and Gynaecology, The University of Western Australia, Perth, WA, Australia Email: lisa.stinson@uwa.edu.au

Background Early‐life microbial colonisation is believed to play a role in immune programming and health, yet the evidence regarding the origins, timing and significance of the neonatal microbiome remains inconclusive due to concerns around sample integrity, confounding factors & contamination.

Methods Placental, amniotic fluid, first‐pass meconium and cord blood samples were collected from 50 elective Caesarean section deliveries. An optimised sampling and analysis protocol was employed to minimise/control for contamination, allowing characterisation of the fetal gut microbiome and its relationship with maternal parameters, short chain fatty acid (SCFA) levels and inflammation.

Results All meconium samples contained detectable levels of bacterial DNA and the immunomodulatory SCFAs acetate and propionate, confirming the hypothesis that the fetal gut is inoculated with bacteria in utero. At the phylum level, meconium was dominated by Proteobacteria and Firmicutes. Acinetobacter ‐ known to be allergy protective ‐ was the most abundant genus (found in 95% of samples), followed by Pelomonas (88%). Importantly, these two genera have been found in the core non‐pregnant endometrial microbiome, but not paired vaginal samples. Lactobacillus (which dominates the vaginal microbiome) was found in only 5 samples. Maternal diabetes, intra‐amniotic inflammation and SCFA levels were all associated with altered meconium microbiome.