The human microbiome and the great obstetrical syndromes: a new frontier in maternal-fetal medicine.

The emergence of the concept of the microbiome, together with the development of molecular-based techniques, particularly polymerase chain reaction (PCR) amplification using the 16S ribosomal RNA (rRNA) gene, has dramatically increased the detection of microorganisms, the number of known species, and the understanding of bacterial communities that are relevant to maternal-fetal medicine in health and disease. Culture-independent methods enable characterization of the microbiomes of the reproductive tract of pregnant and nonpregnant women, and have increased our understanding of the role of the uterine microbiome in adverse obstetric outcomes. While bacterial ascent from the vaginal tract is recognized as the primary cause of intrauterine infection, the microbiomes of the gastrointestinal, oral, and respiratory tracts are shown to be involved by means of hematogenous spread. The transmission of maternal microbiomes to the neonate, by vaginal delivery or cesarean section, is shown to affect health from birth to adulthood.