Preterm birth, which occurs before 37 weeks of pregnancy, is a leading cause of infant deaths globally. Now, a new study suggests that a mother’s gut bacteria can influence preterm birth by breaking down key pregnancy hormones, and this effect interacts with her genetic risk.

The findings, published in Cell Host & Microbe, indicate that gut microbes can interact with genetics and hormone metabolism to influence pregnancy outcomes.

Recent studies have suggested that a mother’s gut bacteria can affect pregnancy health, but while vaginal bacteria have been linked to early birth, the impact of gut microbes is less clear. 

Some gut bacteria can break down hormones such as estradiol, which helps maintain pregnancy and trigger labor, suggesting that interactions between gut bacteria, genetics, and hormones may influence preterm birth risk.

So, researchers led by Zelei Miao at Westlake University in Hangzhou, China, analyzed 5,313 pregnant Chinese women to understand how gut bacteria and genetics influence preterm birth.

Microbial risk score

The team collected stool and blood samples from about 4,200 women, along with clinical and lifestyle data. The overall composition of gut bacteria was linked to how long pregnancies lasted and whether babies were born prematurely. 

These findings were confirmed in a second group of about 1,000 pregnant women, suggesting that the maternal gut microbiota plays an important role in pregnancy outcomes. 

By combining information from several gut bacteria linked to preterm birth, the researchers developed a “microbial risk score”, finding that women with higher risk scores were more likely to deliver early than those with lower risk scores. While both a mother’s genes and gut bacteria influenced risk, the gut microbiota had a stronger impact on predicting early delivery.

Predicting preterm birth

Next, the researchers combined the microbial risk score, the genetic risk, and other risk factors such as age to create a model that could predict preterm birth more accurately than conventional methods alone. 

Women with higher levels of Clostridium innocuum bacteria had an increased risk of preterm birth, which was amplified in those with higher genetic susceptibility. Further analyses showed that C. innocuum can degrade estradiol—a hormone essential for pregnancy. 
“These findings point to future strategies to prevent and mitigate preterm birth by specifically monitoring C. innocuum levels during early pregnancy and developing interventions targeting estradiol metabolism,” the authors say.