Returning to the sterile womb hypothesis

It has long been considered that the human womb is sterile in healthy pregnancies, but recent studies have suggested that the human placenta has its own microbiota. An accurate interrogation of the question requires a sufficiently large sampling size, including different birth outcomes from vaginal versus Caesarean section deliveries, as well as rigorous experimental controls to determine concordance between methods and identify sources of potential contamination. Marcus de Goffau and colleagues now report the results of such a study, and find that the human placenta does not have a microbiota, but the opportunistic pathogen group B Streptococcus is found in approximately 5% of samples. These findings suggest that bacterial infection of the placenta is not a common cause of adverse pregnancy outcomes, and that the human placenta is typically sterile.