Pregnant women infected with certain strains of human papillomavirus (HPV) may have a heightened risk of preterm birth, a new study suggests.
HPV is a sexually transmitted disease that can cause genital warts. Most of the time, the immune system clears the infection. But some strains of HPV become persistent in a minority of people -- and, over time, may lead to certain cancers.
In fact, nearly all cervical cancers in the United States are caused by persistent HPV infection, according to the U.S. Centers for Disease Control and Prevention. And two strains of HPV -- known as 16 and 18 -- are responsible for most.
Now the new study, published online Sept. 15 in the journal JAMA Network Open, links the two strains to preterm birth.
Researchers found that of 899 pregnant women, those infected with the strains throughout pregnancy were almost four times more likely to deliver prematurely as uninfected women were. Premature births occur before the 37th week of pregnancy.
The findings do not prove cause and effect, said senior researcher Helen Trottier, an assistant professor at the University of Montreal in Canada.
There could be other explanations for the link between high-risk HPV and preterm birth, according to Trottier. Her team accounted for as many alternative explanations as they could -- including women's smoking and drinking habits, their age and education level, and whether they had pregnancy-related high blood pressure or diabetes.
But persistent infection with HPV-16/18 was still, itself, a risk factor for preterm delivery.
"It may be that the trouble is with HPV-16 and -18 specifically," Trottier said. That would not be surprising, she added, since those are the variants most often causing lesions in the cervix.
That raises another question, though: When Pap testing for cervical cancer detects a more-severe cervical lesion, doctors may remove the abnormal cells. So it's possible that treatment, which alters the cervix, could explain the connection between high-risk HPV and preterm birth.
Trottier's team did not find evidence of that, however: The link was just as strong among women with no history of cervical treatment, they found.
Still, more research is needed to understand what's going on, according to Trottier. "This is the first study showing this," she said. "We need more studies to confirm the finding."
HPV is the most common sexually transmitted disease in the world -- so common that nearly all sexually active people will contract the virus if they don't get the HPV vaccine, according to the CDC.
The new findings illustrate that. Of the 899 pregnant women involved, 42% tested positive for HPV in their first trimester, and for most the infection persisted through the third trimester. (There is no treatment that eradicates the virus, Trottier noted.)
About 7% of women harbored the HPV-16 or -18 strains, according to the report.
Ultimately, 55 women gave birth prematurely. At first glance, there was no link between preterm birth and HPV infection. But when the researchers zeroed in on the type of HPV, the picture changed.
Women with persistent HPV-16 or -18 infection were 3.7 times more likely to deliver early, versus women with no HPV infection in the first and third trimesters.
Dr. Joy Baker is an obstetrician/gynecologist and spokesperson for the non-profit March of Dimes.
She called the new findings "very interesting." Certain sexually transmitted infections are known to be associated with preterm birth, Baker said, namely bacterial infections that can cause irritation in the cervix. But HPV, a viral infection, has not been on the radar.
Baker said it will be important for future studies to confirm the finding.
If the HPV strains do contribute to preterm birth, Trottier said, that would be critical, in part because infection can be prevented with the HPV vaccine.
In the United States, the vaccine is recommended for girls, as early as age 9 and by age 12. It's also recommended for boys, as HPV can cause cancers of the penis, anus and throat.
For now, Baker said there are some established, modifiable risk factors for preterm birth -- including being underweight or overweight before pregnancy, or having high blood pressure or diabetes before or during pregnancy.
Another contributor, Baker noted, is having a short interval between pregnancies. It's recommended that women wait 12 to 18 months after delivery before becoming pregnant again.
Early prenatal care is key, she added. But ideally, women should see their doctor when they are planning a pregnancy to get a health check-up.
"Make time for those preventive visits," Baker said. "Learn about your health, and try to plan your reproductive life."
The March of Dimes has more on preterm birth.
SOURCES: Helen Trottier, PhD, MSc, assistant professor, department of social and preventive medicine, University of Montreal, Quebec, Canada; L. Joy Baker, MD, spokesperson, March of Dimes, Arlington, Va.; JAMA Network Open, Sept. 15, 2021, online