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The placental microbiome is the nonpathogenic, commensal bacteria present in a healthy human placenta and is distinct from bacteria that cause infection and preterm birth in chorioamnionitis. Until recently, the healthy placenta was considered to be a sterile organ but now genera and species have been identified that reside in the basal layer.
The placental microbiome more closely resembles that of the oral microbiome than either the vaginal or rectal microbiome.
Culturable and non-culturable bacterial species in the placenta obtained following normal term pregnancy have been identified.
A change in the composition of the microbiota in the placenta is associated with excess gestational weight gain, pre-term birth, and decreased overall species richness and variant abundance. The placental microbiota varies between low birth weights and normal birth weights. While bacteria are often found in the amniotic fluid of failed pregnancies, they are also found in particulate matter that is found in about 1% of healthy pregnancies.
In non-human animals, part of the microbiome is passed onto offspring even before the offspring are born. Bacteriologists assume that the same probably holds true for humans.
Investigations into reproductive-associated microbiomes began around 1885 by Theodor Escherich. He wrote that meconium from the newborn was free of bacteria. This was interpreted as the uterine environment was sterile. Other investigations used sterile diapers for meconium collection. No bacteria were able to be cultured from the samples. Bacteria were detected and were directly proportional to the time between birth and the passage of meconium. A 1927 study demonstrated the presence of bacteria in the amniotic fluid of those that were in labor for longer than six hours.