Is amniotic fluid of women with uncomplicated term pregnancies free of bacteria?

The "sterile womb" paradigm is debated. Recent evidence suggests that the offspring's first microbial encounter is before birth in term uncomplicated pregnancies. The establishment of a healthy microbiota early in life might be crucial for reducing the burden of diseases later in life.We aimed to investigate the presence of a microbiota in sterilely collected amniotic fluid in uncomplicated pregnancies at term in the Preventing Atopic Dermatitis and Allergies in children (PreventADALL) study cohort.Amniotic fluid was randomly sampled at cesarean deliveries in pregnant women in 1 out of 3 study sites included in the PreventADALL study. From 65 pregnancies at term, where amniotic fluid was successfully sampled, we selected 10 from elective (planned, without ongoing labor) cesarean deliveries with intact amniotic membranes and all 14 with prior rupture of membranes were included as positive controls. Amniotic fluid was analyzed by culture-independent and culture-dependent techniques.The median (min-max) concentration of prokaryotic DNA (16S rRNA gene copies/mL; digital droplet polymerase chain reaction) was low for the group with intact membranes [664 (544-748)]-corresponding to the negative controls [596 (461-679)], while the rupture of amniotic membranes group had >10-fold higher levels [7700 (1066-251,430)] (P = .0001, by Mann-Whitney U test). Furthermore, bacteria were detected in 50% of the rupture of amniotic membranes samples by anaerobic culturing, while none of the intact membranes samples showed bacterial growth. Sanger sequencing of the rupture of amniotic membrane samples identified bacterial strains that are commonly part of the vaginal flora and/or associated with intrauterine infections.We conclude that fetal development in uncomplicated pregnancies occurs in the absence of an amniotic fluid microbiota and that the offspring microbial colonization starts after uterine contractions and rupture of amniotic membrane.