What Happens to the Vaginal Microbiome During Pregnancy?

If you’re currently pregnant or have been so before, you’ve likely noticed the anticipated changes in the way your body looks and feels: the things you crave (and those you can’t stand), physical symptoms, and the patterns of your emotional fluctuations.

But did you know that your vaginal microbiome is also experiencing its own changes during pregnancy?

As pregnancy progresses, your genital tract undergoes a number of changes that can affect the microbes within it. Here are a few: shifting hormone levels, altered vaginal mucus production, and changing daily behaviors can all impact your vaginal microbiome during pregnancy. 

These changes aren’t a one-way street, though: emerging research indicates that the vaginal microbiome may also impact pregnancy and birth outcomes. While there’s still considerable work to do in this field, let’s discuss what science has shown us so far.

The First Trimester: A Time of Change

The early months of pregnancy are characterized by a gradual shift to a more homogeneous, stable population of vaginal microbes.

Throughout the first trimester, lactobacilli species come to dominate the vaginal microbiome. Remember, lactobacilli are the local heroes of the vaginal microbiome, working to ensure the vaginal environment is inhospitable for potential pathogens. 

Fun fact: this progression toward an increasingly stable microbiome occurs simultaneously in the gut and mouth.*

For some pregnant people, this shift is complete by the end of the first trimester; for others, it takes a bit longer. 

Early-stage research suggests that this transition towards stable Lactobacillus dominance may be protective, as some studies have found a correlation between high-diversity vaginal microbiomes (aka not stable, Lactobacillus dominated) and preterm (early) births:

• In a group of mostly White and Hispanic pregnant women, vaginal microbiomes low in Lactobacillus were correlated with earlier delivery.
• A 2017 study combined data from four previous studies of more than 300 pregnant women. The authors found that a high diversity of vaginal microbes during the first half of pregnancy correlated with higher rates of preterm birth.

Bear in mind that the research has demonstrated correlations, not causations. 

Current research has not established that having a high-diversity vaginal microbiome will definitely cause preterm delivery. However, there appears to be a relationship between the microbiome and delivery outcomes, and hopefully, future data and research will demystify this connection (we’re working on it!).

Interestingly, another study of predominantly Black women reported opposing results. In this population, a decrease in microbiome diversity between the first and second trimesters was associated with preterm birth. In this case, a switch to a higher diversity of microbes was linked to full-term deliveries (contrary to studies in other populations).

However, you could interpret this data in a slightly different way: the authors found that change in the vaginal microbiome between the first and second trimesters was associated with preterm birth. So it is still unclear if the overall diversity of the microbiome or the amount of change in the microbiome during pregnancy is what is leading to an increased risk of preterm birth. 

In yet another study of full term healthy pregnancies in Black women found that the majority decreased in diversity overall and shifted towards a Lactobacillus (primarily L. iners) dominant microbiome.  

So what does all this mean? Essentially, this tells us that more robust studies in more diverse populations are necessary to determine whether vaginal microbiome diversity will be a useful predictor of preterm birth. 

*Note: this research study was conducted in a predominantly White and Hispanic sample population. Therefore, differences may exist for people of different racial/ethnic backgrounds.

The Second Trimester: The Cervix and Stability in the Vaginal Microbiome 

While racial differences in the vaginal microbiome predominate during the first trimester, the second trimester appears to be the home of homogeneity. By the second trimester, studies indicate that women of different races tend to have a stable, low-diversity vaginal microbiome dominated by Lactobacillus. 

In addition to a changing microbiome, the vagina is shifting in other ways during pregnancy, particularly with regard to the cervix.  

The cervix is the lower end of the uterus and it forms a protective barrier between the uterus and vaginal canal. It may change in length during pregnancy, and a short cervix is an established risk factor for premature birth. However, researchers don’t yet fully understand the factors that contribute to the shortening of the cervix. 

In 2020, a research group in Philadelphia reported a number of interesting connections between the vaginal microbiome, cervical shortening, and preterm birth.

This group studied 472 pregnant women, measuring cervical length via ultrasound and collecting swab samples to test the vaginal microbiome. 

Here were the results: 

• They found an association between a short cervix and an increased likelihood of preterm birth. 
• They determined that a highly diverse vaginal microbiome that is low in Lactobacillus (aka community state type IV) increased a pregnant woman’s odds of developing a short cervix.

• This particular study found that women with type IV microbiomes in this study were more likely to be younger and Black.

• They found that women with a short cervix AND a highly diverse vaginal microbiome were more likely to deliver early. This combination increased the odds of preterm birth more than either one alone.

The same research group published a follow-up study in 2022 that attempted to determine the causes of a short cervix during pregnancy. They found that changes in the lipids (fats) on the surface of the cervix were associated with cervical shortening. This research, along with further data on the vaginal microbiome, may help scientists determine new risk factors for preterm birth. 

The Third Trimester and the Postpartum Transition

During the third trimester of pregnancy, the vaginal microbiome typically remains quite stable, dominated by Lactobacillus species. A 2015 study found that vaginal microbiomes remained relatively stable throughout pregnancy, especially for those with community state types (CSTs) dominated by protective Lactobacillus. 

Interestingly, while some of the people in this study transitioned between multiple CSTs throughout pregnancy, these transitions didn’t seem to predict their birth outcomes (i.e., whether they delivered early or at full term).

Another study published in 2021 analyzed the microbiomes and other molecules (biomarkers) in vaginal samples from 48 pregnant, mostly White women. The researchers found that both the microbiomes and the biomarker molecules were similar during the first and third trimesters of pregnancy. 

This research appears to contradict the previous studies that showed substantial changes in the vaginal microbiome during early (first trimester) pregnancy. These discrepancies highlight the need for further studies in larger, diverse populations to better determine the importance of the vaginal microbiome on pregnancy and childbirth.

So what does this mean for people who are pregnant right now? 

While the science is still developing, this doesn’t mean that you have to wait to better understand your own vaginal health and how your microbiome may be shifting during pregnancy. 

The Evvy membership can help you test your microbiome periodically throughout pregnancy so you can monitor for changes and stay ahead of them with your healthcare provider.

The Postpartum Period: A Slow Transition Back to Baseline

The third-trimester studies we previously mentioned both noted a significant shift in the vaginal microbiome after delivery. They noted that the postpartum microbiome transitioned away from Lactobacillus dominance and towards a more diverse population of microbes. 

Specifically, the 2015 study found that this more diverse microbiome persisted for at least one year postpartum in some women, and possibly even longer. 

These findings may be relevant to the birth outcomes of people who have short intervals (<12 mo) between pregnancies. Additionally, a more diverse microbiome that is low in protective Lactobacilli may predispose postpartum people to vaginal infections during this period. 

If you’re concerned about the health of your vaginal microbiome after delivery, Evvy is here to help take one worry off your plate while you care for your newborn.  

Evvy tests for all bacteria and fungi that can impact the vaginal microbiome,  and it can help you determine how best to care for your vagina after birth. If your results indicate your microbiome is out of whack after baby, Evvy’s vaginal healthcare can help you bring it back into balance.

Race, Pregnancy, and the Vaginal Microbiome 

Research on the vaginal microbiome is continually expanding, and scientists don’t yet fully understand what causes racial differences in the microbiome. 

Because such extensive racial discrepancies exist in pregnancy and delivery (i.e., Black women are 3x more likely to die from a pregnancy-related cause than White women), we feel it’s essential to cover racial differences in the vaginal microbiome during pregnancy. 

Women of African ancestry will experience more significant shifts in their vaginal microbiome during pregnancy, typically during the first trimester. And while they do tend to shift towards a low diversity Lactobacillus dominant microbiome, the species of Lactobacillus is different.

It’s clear that there’s still a lot to learn about vaginal health and its impacts on pregnancy. Continued research on diverse pregnant populations will help us better understand the impact of race on the vaginal microbiome and pregnancy outcomes.