The vaginal microbiome during pregnancy is more stable than that of non-pregnant women. Women delivering at full-term gestation had a Lactobacillus dominant vaginal microbiome.

Pregnancy is a human-body changer. Ask any woman who’s gone through it. My hair went from light blonde to this mousy blonde after my first trimester with my first baby, Jac. Never mind that baby belly I don’t think I’ll ever lose. That pooch is part of me now. Though I have to say, pregnancy was the one time in my life where my allergies and eczema weren’t a problem. So what about your microbial self? Your microbiome? Do all body sites have a more or less stable microbiome during pregnancy? Or do some body sites, like gut or vaginal microbiomes, change during pregnancy? Since birth seems to be the primary time that microbes are passed from mother to offspring, perhaps the vaginal microbiome would change as birth approaches.

Vaginal Community State Types

The Ravel lab sampled the vaginal fluid microbiome community throughout the pregnancy of 22 women who delivered babies at term [1]. The vaginal fluid microbiome of pregnant and non-pregnant women was compared. What the researchers found was that the vaginal bacterial communities of both pregnant and non-pregnant women did not change between community state type. A woman who has a vaginal microbiome dominated by Lactobacillus inners will continue to keep that microbiome community throughout pregnancy.

Community state types (CST) are a way to describe the total bacterial community of a given body site. The Ravel lab had described 5 different vaginal microbiome CST’s in an earlier paper [2]. In four of these CSTs the microbiome is dominated by one of 4 different species of LactobacillusLactobacillus iners, L. crispatus, L. gasseri, or L. jensenii. The fifth CST was characterized as having a mix of anaerobic bacteria and some Lactobacillus. Bacterial community state type is similar to my dining room’s fruit bowl diversity. My husband enjoys grocery shopping (thank goodness) and as a botanist, he likes to pick up different kinds of fruits. The kids prefer just apples and pears. I can tell if the kids went grocery shopping with Daddy or not depending on the “community state” of the fruit bowl. Community state A (hubby’s selection) might have Golden apples, clementines, bananas, grapes, and plums. Community state B (kid’s choice) has red Gala apples and pears. In both cases, apples are present, but there are other fruits. Community state A is the more diverse community with 5 different kinds of fruits. Community state B has just 2 kinds of fruits.

ravel 2014 Fig1

Vaginal Microbiome Fluctuates with Menses

In the microbiome study of pregnant women, a few women did switch between community states. However, switching wasn’t common. A woman who has a mixed community microbiome will continue to have a mixed community microbiome. Those women with L. crispatus dominated CST’s continued to have that same CST. In fact, the vaginal communities are in pregnant vs non-pregnant women and found that the vaginal community was MORE stable in pregnant women than non-pregnant. These findings aren’t that surprising since this same research group found that the vaginal microbiome fluctuates with hormone changes throughout menses. When estrogen concentrations are high, microbial communities are most stable. This stability might be due to estrogen increasing mucus associated with the vaginal epithelium. Lactobacillus sp. use vaginal mucus to make lactic acid that lowers the vaginal pH and helps prevent pathogens [3].

Is Lactobacillus sp. Important for Full Term Delivery?

The vaginal microbiome of pregnant women can change between the Lactobacillus-dominant community state types [1]. Instead of Lactobacillus crispatus dominated microbiome, a woman might have a L. iners dominated microbiome. Women didn’t change between one of the 4 Lactobacillus dominant and the 5th – mixed community state type [1]. So this would be like the kids changing their apple and pear community state. Community State B has Gala apples. Community state C has Granny smith apples. The fruit are still apples, not grapes and clementines. Also during pregnancy, the bacterial community does change the number of different kinds of bacteria within that CST. Back to the fruit bowl example, we might be in community state B where there are only Gala apples and pears, but the structure of the fruit bowl might change. Maybe one week there are 6 apples and 2 pears, another week there are 4 apples and 4 pears. It’s the same community type B – apples and pears, but the numbers of the different individuals of that species have changed.

Ravel 2014 fig2

Next, they looked to see if pregnant and non-pregnant women had different microbiomes overall. What they found was that pregnant microbiome were more likely to be Lactobacillus dominated than non-pregnant women. Rarely did pregnant women who delivered at term have species of bacteria that are associated with bacterial vaginosis [1].  Instead, their vaginal microbiomes were more likely to be dominated by L. vaginalis, L. jensenii, L. crispatus, and L. gasseri. This finding was especially striking since over 90% of the women sampled in the research were black women. White and Asian women have vaginal microbiomes that are dominated by species of Lactobacillus. Hispanic and Black women were more likely to have vaginal microbiomes dominated by a mix of anaerobic bacteria [2]. While Lactobacillus might be present, it was only a small part of the microbiome. Do Hispanic and black women with a mixed vaginal microbiome BEFORE pregnancy switch to a Lactobacillus-dominated vaginal microbiome DURING pregnancy? If so, what influences this microbiome CST change? Does that Lactobacillus-dominant microbiome specific role or is it due to physiology changes, like hormones, due to pregnancy? Like so much of the human microbiome research, new findings constantly change how we think of our human-microbial self. Each new study is another baby step forward in our understanding.




  1. Romero R, Hassan SS, Gajer P, Tarca AL, Fadrosh DW, Nikita L, Galuppi M, Lamont RF, Chaemsaithong P, Miranda J et al: The composition and stability of the vaginal microbiota of normal pregnant women is different from that of non-pregnant women. Microbiome 2014, 2(1):4.
  2. Ravel J, Gajer P, Abdo Z, Schneider GM, Koenig SS, McCulle SL: Vaginal microbiome of reproductive-age women. Proc Natl Acad Sci U S A 2011, 108.
  3. Gajer P, Brotman RM, Bai G, Sakamoto J, Schütte UME, Zhong X, Koenig SSK, Fu L, Ma Z, Zhou X et al: Temporal Dynamics of the Human Vaginal Microbiota, Sci Trans Med vol. 4; 2012.

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