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Not Much Bacteria Found

The human placenta might actually be sterile, according to a new study.

Researchers from the University of Cambridge and elsewhere studied placental biopsies from more than 500 women, some of whom had adverse pregnancy outcomes like preeclampsia, spontaneous pre-term birth, or newborns who were small for their gestational age. As they report in Nature this week, the researchers found little evidence of bacteria in any of the placental samples.

Instead, they suggest that previous reports of a placental microbiome could instead be chalked up to acquiring bacteria during labor and delivery or the contamination of lab tools. "We are not the first to say this, but all the signals [of a microbiome] that you find in the placenta are due to contamination," study author Julian Parkhill from Cambridge tells New Scientist.

Baylor College of Medicine's Kjersti Aagaard, who reported in 2014 on the presence of a placental microbiome, says that its possible existence cannot yet be dismissed. She tells The Atlantic that the approach the Cambridge-led team took was unlikely to find any bacteria and may have been too stringent in its filtering of ones it did detect.