Vaginal microbiome once again tied to preterm birth

Preterm birth is major global health challenge.  Today, around 11% of all babies are born prior to 37 weeks, and are considered preterm.  Many of the causes of these preterm births are still unknown, but it is thought that around 25% of them may be related to a bacterial infection that comes from somewhere in the mother’s own body, i.e. her microbiome.  Many studies are now linking specific vaginal bacteria to risk of preterm birth, and other studies have even shown a connection between other microbiome sites, such as the gut and oral microbiome.  Unfortunately, studies on the microbiome and preterm birth are extremely difficult to conduct, so there are just not enough to have any sort of scientific consensus on the topic.  Last week though, a very rigorous study out f Stanford University was published in the Proceedings of the National Academy of Sciences that monitored expectant mothers vagina, gut, and oral microbiome throughout the course of her pregnancy and then for one year after.  Among many interesting findings, which are discussed below, the most important one was yet another connection between bacterial vaginosis and preterm birth.

The researchers monitored the vaginal, distal gut, salivary, and tooth/gum microbiomes of 49 women, 15 of which ended up delivering preterm, over the course of their pregnancy and for one year after.  Interestingly, the non-vaginal sites’ microbiomes remained relatively stable over the duration of the pregnancy, and even for the one year after.  The vaginal microbiome, however, did show some differences during and after pregnancy.

As many of our readers already know, a healthy vaginal flora is dominated by Lactobacilli, but around 20% of American women are dominated by other species, such as Gardnerella vaginalis, and have an overall increased vaginal diversity.  These women have what is known as community state type four, or CST4, and these women could be diagnosed with bacterial vaginosis (BV), though the clinical diagnosis is not so specific.  The other community state types, CST1, 2, 3, and 5, are dominated by different strains of Lactobacilli, and are generally regarded as healthy.  This current research showed that many of the women’s vaginal microbiomes actually shifted between various CST’s during pregnancy, most often shifting to and from CST4.  These transitions had no association with preterm birth, though.  After giving birth the vaginal microbiome became more diverse, and had greater abundances in anaerobic bacteria, such as Peptoniphilus, Prevotella, and Anaerococcus.  In addition, this usually coincided with a decrease in Lactobacilli.  Surprisingly, these changes did not seem to relate to mode of delivery (C-section of vaginal).

CST4 has been linked to preterm birth before, and this was reinforced in this study.  The scientists found that the longer a women’s vaginal microbiome was within CST4, the greater risk she had for preterm birth.  In addition, the abundance of Gardnerella and Ureaplasma, specifically, were linked to preterm birth.

This study reinforces what many microbiome scientists already suspect, and that is the importance of the vaginal flora in preterm birth.  It is unclear at this point if manipulating the vaginal flora prior to, or during pregnancy would help prevent preterm birth, but it is certainly worthy of discussion and clinical testing.  If you are reading this and wondering what your vaginal microbiome is, then we recommend you participate in the citizen science project, YourPrivateBiome, to find out.  You can learn more about it by following this link on our site, or just click the link above.

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