Contrary to the common idea of a “sterile” intrauterine environment, Aagaard and coauthors demonstrated the presence of a microbiome in placentas from 320 healthy pregnancies, collected under sterile conditions. This microbiome was quite different from that reported in other parts of the mother´s body including the vagina and the gut, and resembled more the microbiome of her oral cavity, however in much lower abundance.
This raises the question – without being able to answer it yet – whether bacteria ingested via food of mothers are able to affect the microbiome development of the baby even before delivery. An altered placental microbiome was associated with antenatal oral infections, that in turn may have resulted in preterm deliveries.
Source: Kjersti Aagaard et al. The Placenta Harbors a Unique Microbiome. Sci Transl Med 6, 237ra65 (2014)