CIDRAP-Mothers vaccinated against HPV before pregnancy less likely to give birth prematurely, study finds

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Mothers vaccinated against HPV before pregnancy less likely to give birth prematurely, study finds

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Mothers vaccinated against HPV before pregnancy less likely to give birth prematurely, study finds​

Liz Szabo, MA

Today at 12:55 p.m.
Human Papillomavirus (HPV)


Mothers vaccinated against the human papillomavirus (HPV) before pregnancy are less likely to develop pregnancy complications, especially if immunized before age 17, a large new study finds.

Researchers found that women immunized against HPV were 15% less likely than women unvaccinated against HPV to deliver very prematurely, or during the 28th to 31st weeks of a 40-week pregnancy, according to a Swedish study published this week in The BMJ. Babies born before the 37th week of pregnancy are more likely to have poor health or disabilities.

Mothers immunized against HPV also were:

• 5% less likely to give birth before the 37th week
• 7% less likely to have a rupture of the amniotic membrane before labor, which increases the risk of infection unless the baby is delivered right away
• 8% less likely to have a baby that’s much smaller than the expected size for its stage in pregnancy

The rate of pregnancy complications was lower in women immunized against HPV before age 17, likely because the women had not yet been exposed to the virus, which is spread through sexual contact.

Among women vaccinated before age 17, the risk of premature delivery was 10% lower than among unvaccinated mothers. In addition, the risk of a premature member rupture was 14% lower, and the risk of a baby being small for their gestational age was 9% lower.

The study drew on more than 624,000 birth records from mothers ages 16 to 35 years from 2006 through 2023.

Future studies may provide more answers​

Future studies may reflect even greater protection from HPV vaccines, said Zhongsong Zhang, first author of the paper and a PhD student in medical epidemiology and biostatistics at the Karolinska Institute in Stockholm.

About 15% of mothers in the study were vaccinated against HPV. More than 90% of Swedish girls born in 2012 have received at least one dose of HPV vaccine.

HPV vaccines also have improved in recent years.

Mothers in the study received an earlier version of the HPV shot that protected them against four strains of the virus. HPV vaccines have since been updated to protect against nine strains, including seven viral subtypes that cause cancer, and two that cause genital warts.

Girls are also being vaccinated earlier today than when the study began.In the study, mothers vaccinated against HPV were immunized at a median age of 16. In Sweden today, girls are offered HPV vaccines in school-based programs at ages 10 through 12.

In the United States, the American Academy of Pediatrics recommends children get the first of two doses of HPV vaccine at ages 9 through 12, and 78% of teens received at least one dose in 2024.

Researchers “expect the potential reduction in adverse pregnancy outcomes to become more apparent as cohorts with higher vaccination coverage, particularly those vaccinated at younger ages, reach childbearing age,” Zhang told CIDRAP News. “The stronger association observed among women vaccinated at younger ages is therefore an important finding of our study and also highlights the importance of timely immunization.”

Study confirms results of earlier research​

HPV is a ubiquitous virus that causes six types of cancer.

The HPV vaccine has been shown to reduce the risk of cervical cancer by 80%. People immunized against HPV are also less likely to develop vaginal and vulvar precancers, as well as oral, penile, anal, and esophageal cancers.

Earlier population-based studies in Australia, Finland, and Scotland also have linked HPV vaccines and healthier pregnancies. The new study, however, “is the largest and most comprehensive” research yet, said Kevin Ault, MD, a professor and vice chair of education at Penn State Health, who was not involved in the study.

The new findings make biological sense, said Ault, an obstetrician-gynecologist.

In the new paper, researchers noted that previous research found that mothers infected by HPV are nearly twice as likely to deliver prematurely. Mothers with HPV are also more likely to have babies with a low birth weight (less than 5 pounds, 8 ounces).

“Getting vaccinated before pregnancy reduces this risk,” Ault said.

Possible reasons for better pregnancy outcomes​

HPV may put pregnancies at risk in multiple ways, said Rebecca Perkins, MD, obstetrician and gynecologist and investigator at the Woman, Mother and Baby Research Institute at Tufts Medical Center, who wasn’t involved in the new study.

Chronic HPV infection can cause cervical cancers or precancers. Removing these growths can shorten the cervix, which increases the risk of pregnancy complications, Perkins said.

Research shows that even a biopsy of the cervix, in which a doctor removes a cone-shaped area of tissue to examine under the microscope, increases the risk of premature delivery, premature membrane rupture, and low infant birth weight.

“Surgery on the cervix is a well-known risk factor for preterm birth,” Ault said.

Yet the study shows that even mothers who had never been treated for cervical lesions had higher risks of pregnancy complications if they weren’t vaccinated against HPV.

That suggests “that infection alone carries risks,” Perkins said. “Infection-related risks may arise from inflammation or placental dysfunction.”

Ault noted that HPV may increase the risk of pregnancy complications by impairing the immune system.

Immunologic studies have found that “HPV-infected cells in the vagina and cervix have blunted responses to infections,” Ault said. “You don’t have to have precancer or cancer to have these blunted responses.”

Study doesn’t prove cause and effect​

Mark Einstein, MD, chair of obstetrics and gynecology and women’s health at Montefiore Medical Center in New York, said he’s not convinced that HPV vaccines truly improve pregnancy outcomes.

Because of the study’s design, it’s not possible to definitively conclude that HPV vaccination reduced the risk of pregnancy complications, the authors acknowledged in the article.

It’s possible that something other than the vaccine itself led women immunized against HPV to have healthier pregnancies, Einstein said. If so, that could skew the results, making immunizations appear more beneficial than they really are.

“It could be that those that seek HPV vaccination are more likely to be from the social and racial demographics that put them at lower risk of preterm birth,” Einstein said.

The authors tried to eliminate outside influences on the study results in several ways.

They compared each mother who experienced a pregnancy complication with up to 10 women of the same age and year of conception who didn’t have a pregnancy complication. The researchers also considered other factors that could influence the health of a pregnancy, such as cigarette smoking, body mass index at the first prenatal visit, maternal health issues, the mother’s country of birth, family income, education level, and whether women used assisted reproduction.

Combined with studies finding no increase in miscarriage rates after HPV vaccination, the new study’s results help to refute false claims that HPV shots harm girls’ future fertility.

“Together with previous research showing the safety of human papillomavirus vaccination during pregnancy, our results suggest that human papillomavirus vaccination is not only safe with respect to pregnancy outcomes, but may also confer additional benefits by reducing the risk of adverse events,” the study authors wrote.