During pregnancy, a person’s body undergoes a lot of changes – hormone fluctuations, morning sickness, loosening joints and ligaments, and well, the obvious: growing a mini human. As a microbiologist, I’ve wondered about the microbiome. What changes have occurred in the vaginal microbiome? What do these changes mean? And can the vaginal microbiome be predictive of the “health” of the pregnancy?
What is the Vaginal Microbiome?
You’ve probably heard of the gut microbiome, but the vaginal has a microbiome as well. In fact, the vagina is home to between 10 to 100 billion bacteria. In a healthy vaginal microbiome, most of the bacteria are Lactobacilli, which help protect the vagina from pathogens by producing lactic acid, antimicrobial peptides, and hydrogen peroxide.
The vaginal microbiome encounters many changes during menstruation and pregnancy. And sometimes, the vaginal microbiome becomes imbalanced, for example, during vaginosis or yeast infections.
The Vaginal Microbiome Community State Types
Vaginal microbiomes fall into one of five different “community state types” or CST. These CSTs are categorized based on the Lactobacillus species present and their abundance. The different CSTs also are associated with different vaginal health outcomes – some being protective (ex: CST I, II, and V) while others are disruptive (CST IV). (Find more information on community state types here.)
The Microbiome During the Course of Pregnancy
The first longitudinal study of the vaginal microbiome over the course of pregnancy was done in 2014 by Jacques Ravel’s lab. The lab received samples every four weeks until 24 weeks of gestation and every two weeks until the last prenatal visit. For non-pregnant people, samples were collected twice weekly for 16 weeks.
By comparing samples from pregnant people to those of non-pregnant people, they found that:
- Bacteria associated with bacterial vaginosis was rarely found in pregnant people who delivered at full term.
- The vaginal microbiome of pregnant people who delivered at full term vs. non-pregnant people had higher abundance of Lactobacillus vaginalis, L. crispatus, L. gasseri and L. jensenii and lower abundance of dozens of other bacteria from CST IV, which are associated with disruptive health outcomes.
- The vaginal microbiome in pregnant people is more stable than in non-pregnant people. While bacterial communities can shift from one Lactobacillus-dominated CST to another, the community rarely shifts to the CST associated with bacterial vaginosis. The authors of this paper speculate that this increase in stability lends itself to a protective role against infections that can increase the risk for preterm delivery and other conditions.
Another study, done in 2015 by David Relman’s lab, examined how the human microbiome changes during pregnancy by taking weekly microbiome samples during gestation and monthly after delivery from the vagina, gut, saliva, and teeth/gums. Like the Ravel study, they found that the microbiome was stable in the vagina (as well as the other sites) and found that a CST IV microbiome was associated with an increased risk for preterm birth.
Altered Microbiome Associated with Preterm Birth
Expanding the search for a microbial signature of preterm birth, a meta-analysis examined the outcomes from 17 preterm birth microbiome studies published from 2014 to 2021. They found that those with a microbiome low in Lactobacilli were at increased risk for delivering preterm. This is similar to the results from the Ravel study. They also found that microbiomes dominated by L. crispatus (CST I) had the lowest risk for preterm delivery, as this CST is known as protective.
So is it possible that the vaginal microbiome could predict preterm birth? In part, yes, but there’s more to the story.
For example, the immune system undergoes numerous changes throughout pregnancy. For most of the pregnancy, molecules released from the pregnant person prevent the immune system from attacking the embryo. At full term, the immune system emerges from the immune suppressive state and scientists have found that inflammatory proteins called cytokines were at higher levels in the amniotic fluid from people who delivered preterm.
To combine the clues stemming from different sources, scientists have built models that integrate microbiome data from the gut, the vaginal, and the mouth, protein and metabolite data from the blood, and cell counts and gene expression from immune cells in attempts to pin down preterm birth predictors. It’s possible that one day, we will be able to have a test that predicts preterm birth risk based on a blood test and microbiome samples.
The Postpartum Vaginal Microbiome
The Relman lab found that the microbiome was altered in pregnant people for up to a year after delivery. The changes were characterized by a decrease in Lactobacillus species and an increase in anaerobic bacteria like Peptoniphilus, Prevotella, and Anaerococcus species. The researchers noted that the vaginal microbiome more resembled the gut microbiome, but it’s unclear why this is. This shift was observed in both vaginal and cesarean deliveries, suggesting against introduction of gut microbes to the vaginal from stool.
Overall, the microbiome can both impact and be impacted by the many environmental and biological changes during the postpartum period: sleep deprivation, diet, breast infection, and perineal injury, for example. These changes can also affect the newborn’s skin and gut microbiomes.
I’m excited to see the new research on the vaginal microbiome in the upcoming years and to see whether these findings could lead to new tools and diagnostics for our health.
Further Reading
Mom’s immune system and microbiome may help predict premature birth. ScienceNews. 2019.
Temporal and spatial variation of the human microbiota during pregnancy. PNAS. 2015.
What Are the Vaginal Microbiome Community State Types? Evvy. 2022.
Featured image: Lactobacillus acidophilus by Mogana Das Murtey and Patchamuthu Ramasamy.
