HPV Vaccine Linked to Reduced Preterm Birth Risk in Swedish Study

A large Swedish study links HPV vaccination before pregnancy to a reduced risk of adverse pregnancy outcomes, including preterm birth. The nationwide case-control study, published October 7, 2026, in the BMJ, analyzed 624,713 singleton births among first-time mothers aged 16 to 35 between 2006 and 2023.

Statistically significant associations were observed for several key outcomes. Mothers immunized against HPV were 5% less likely to give birth before the 37th week. They were also 7% less likely to experience preterm premature rupture of membranes (PPROM) and 8% less likely to have a baby classified as severely small for gestational age.

Stronger effects seen in those vaccinated before age 17

Protective effects were more pronounced in women who received the quadrivalent HPV shot before turning 17. For this group, the risk of premature delivery was 10% lower compared to unvaccinated mothers. The risk of premature membrane rupture was 14% lower, and the risk of having a severely small-for-gestational-age infant was 9% lower.

About 14.8% of mothers in the study were vaccinated before pregnancy, with a median vaccination age of 16. The study excluded births from women whose first HPV vaccine was the bivalent or nonavalent version, focusing solely on the quadrivalent shot.

Vaccination may reduce risk via prevention of cervical lesions

Researchers suggest the benefits may stem from the vaccine preventing high-grade cervical lesions that can compromise pregnancy. In the study, high-grade cervical lesions before childbirth were far less common in vaccinated women (0.6%) than in unvaccinated women (3.7%). After adjusting for the presence of these lesions, the associations between HPV vaccination and preterm birth outcomes weakened.

Jiayao Lei, PhD, of the Karolinska Institute in Stockholm, told MedPage Today, "This pattern is consistent with the possibility that preventing high-grade cervical lesions and subsequent cervical treatments may partly explain the observed associations." Chronic HPV infection can cause cervical cancers or precancers, and surgical removal of these growths can shorten the cervix, increasing pregnancy complication risk.

Study details and limitations noted

The research is described as the largest and most comprehensive yet on HPV vaccination and pregnancy outcomes. Each case of an adverse outcome was matched with up to 10 control births without that outcome. The authors acknowledge limitations, including potential residual confounding from factors like prior HPV exposure or sexual behavior, especially in women vaccinated at 17 or older. Differences in preventive interventions for preterm birth also could not be fully assessed.

First author Zhongsong Zhang, a PhD student in medical epidemiology and biostatistics at the Karolinska Institute, and senior author Verena Sengpiel, Professor of Obstetrics at the University of Gothenburg, led the work. They caution that the mediation analysis should be interpreted as exploratory rather than definitive evidence of a mechanism.

Broader context of HPV vaccine benefits

The findings add to evidence from Australia, Finland, and Scotland linking HPV vaccines to healthier pregnancies. The HPV vaccine has been shown to reduce the risk of cervical cancer by 80% and protects against other cancers and precancers. In Sweden, girls are now offered the vaccine in school-based programs at ages 10 through 12.

Jiayao Lei and Zhongsong Zhang said, "Although the observed differences were relatively modest at the individual level, they could translate into meaningful public health and economic benefits at the population level." Previous research found that mothers infected by HPV are nearly twice as likely to deliver prematurely and more likely to have low birth weight babies. The new study suggests that preventing high-grade cervical lesions may partly explain the observed associations with improved pregnancy outcomes.