Preterm birth, which occurs before 37 weeks of gestation, afflicts 11% of pregnant women worldwide and has been inconclusively tied to the relative abundances of microbes in and on the human body.

Researchers sampled microbial communities every week of gestation and after delivery in the vagina, stool, saliva, and tooth/gum of 49 pregnant women 18 years of age or older, of whom 15 delivered preterm infants.

Microbiome composition and diversity largely remained stable throughout pregnancy. Vaginal microbiome analysis yielded five major types of communities, four of which were dominated by a different Lactobacillus species. The fifth type, poor in Lactobacillus but rich in diverse species, was associated with preterm delivery in a dose-dependent manner.

In some women with Lactobacillus-poor vaginal microbiomes, an abundance of Gardnerella and Ureaplasma was tied to preterm birth.

Most women displayed an abrupt and persistent change in microbiome composition after delivery, whether vaginal or cesarean, toward a diverse microbiome marked by low levels of Lactobacillus as well as the appearance of anaerobic bacteria such as Prevotella and Peptoniphilus.

According to the authors, microbiomes in early gestational stages might offer clues to the risk of preterm birth and influence maternal health after delivery. 

http://www.pnas.org/content/early/2015/08/12/1502875112