Healthy human placentas are not colonized by microorganisms, according to an analysis of hundreds of placental samples published online in Nature this week. This research supports the longstanding ‘sterile womb’ hypothesis and also suggests that infection of the placenta is not a common cause of adverse pregnancy outcomes.

It has long been thought that the human placenta is a sterile environment free of microorganisms. Previous studies using modern genome sequencing techniques have reported the detection of bacterial DNA in the womb, and it has been suggested that compositions of microorganisms might differ between individuals with healthy and complicated pregnancies. However, other studies have indicated that these detections may be false-positive results due to contamination.

In the largest known study of its kind, Gordon Smith and colleagues analysed placental samples from more than 500 women, using DNA sequencing to search for microorganisms. Their experimental approach was designed to minimize the chance of false-positive results; for example, they used two different kits for DNA extraction, and several molecular methods to detect bacterial DNA. The majority of samples did not display any evidence of bacterial colonization, both in complicated and uncomplicated pregnancies. The exception was group B Streptococcus, a major cause of sepsis in newborns, which was found in approximately 5% of samples.

The findings indicate that the placenta is unlikely to be the main source of the infant microbiota, notes Nicola Segata in an accompanying News & Views. The precise mechanisms by which the human microbiota is established in newborn infants remains to be conclusively determined, but “we can now be confident that the placenta is not the microbial reservoir”, Segata adds.