TL;DR
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A Swedish register-based study found that women vaccinated against HPV before pregnancy had modestly lower odds of preterm birth and some other adverse pregnancy outcomes. The observational findings do not establish that vaccination caused the differences, and the possible role of cervical lesions or treatment remains uncertain.
A study of 624,713 singleton births in Sweden found that women who had received a quadrivalent HPV vaccine before pregnancy had modestly lower odds of preterm birth and some other adverse outcomes than unvaccinated women. The observational study, published in The BMJ, found stronger associations for some outcomes among women vaccinated before age 17, but it cannot establish that vaccination caused the differences.
Researchers led by Jiayao Lei of Karolinska Institute examined nationwide register data for first-time mothers aged 16 to 35 whose singleton births took place from 2006 through 2023. Of the eligible births, 14.8% of women had received a quadrivalent HPV vaccine before pregnancy. The analysis excluded births where the woman’s first HPV vaccination was bivalent or nonavalent.
Compared with women who were unvaccinated, vaccinated women had lower adjusted odds of preterm birth (adjusted odds ratio, 0.95; 95% confidence interval, 0.91 to 0.99), very preterm birth (0.85; 0.75 to 0.97), and spontaneous preterm birth (0.95; 0.91 to 0.99). The researchers also reported lower odds of preterm prelabor rupture of membranes and severe small-for-gestational-age birth. These are relative odds measures; the study does not show that every vaccinated woman’s absolute risk fell by the same amount.
For women vaccinated before age 17, adjusted odds were lower for preterm birth (0.90) and spontaneous preterm birth (0.89) than for unvaccinated women. The study found no significant differences for prelabor rupture of membranes, stillbirth, or neonatal mortality in the overall vaccinated group. Matching each adverse-outcome case with up to 10 controls was used to help address differences between groups, but cannot remove all possible confounding.
Possible Benefits Beyond Cancer Prevention
The findings add evidence to research on whether HPV vaccination may be associated with health outcomes beyond preventing HPV infections and related cancers. If further studies confirm a pregnancy benefit, it could matter to vaccination policy and economic evaluations, which the study’s authors said could account for pregnancy complications and their longer-term healthcare costs as well as cancer prevention.
The reported differences were modest at the individual level, and the study does not show that vaccination should be treated as a method of preventing preterm birth. Its results are associations from an observational analysis, not proof of a protective effect. The immediate relevance is that researchers and public health planners have another outcome to investigate while established HPV vaccination programs remain focused on preventing HPV infection and cancer.
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How Cervical Lesions May Fit In
HPV is associated with cervical lesions, and treatments for high-grade cervical lesions have also been linked in prior research to a higher risk of preterm birth and other pregnancy complications. In this study, a diagnosis of a high-grade cervical lesion before childbirth was recorded for 0.6% of vaccinated women and 3.7% of unvaccinated women.
After researchers adjusted for these diagnoses, the associations between vaccination and preterm-birth-related outcomes weakened. The authors said this pattern is consistent with the possibility that preventing lesions and subsequent cervical treatment could partly explain the findings. They also raised possible direct links between HPV and pregnancy, including effects on embryonic development or the placenta, but the study did not determine which, if any, explanation is correct.
The U.S. schedule cited in the report recommends a two-dose series of the nine-valent HPV vaccine beginning at ages 9 to 12. This study examined the quadrivalent vaccine.
“Our findings highlight the importance of timely HPV vaccination and suggest that its potential benefits may extend beyond cancer prevention to reproductive health.”
— Jiayao Lei and Zhongsong Zhang, Karolinska Institute researchers, speaking to MedPage Today
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Cause and Mechanism Remain Uncertain
The study was observational, so differences in participants’ health, vaccination circumstances, or reproductive histories could contribute to the results. The authors identified possible residual confounding related to catch-up or opportunistic vaccination, earlier HPV exposure, and sexual or reproductive behavior, particularly among women vaccinated at age 17 or older. They also said differences in interventions intended to prevent preterm birth could not be fully assessed.
The analysis did not include detailed records of cervical treatments, making it difficult to determine whether fewer lesions or procedures explain some of the association. The results also do not establish whether HPV itself can affect pregnancy outcomes. The reported odds ratios describe comparisons between groups; they do not provide a direct estimate of how many individual pregnancies vaccination would prevent from ending early.
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Further Studies Must Test the Link
The study’s authors said the findings warrant further consideration in policy and cost-effectiveness analyses, alongside HPV vaccination’s cancer-prevention benefits. They noted that the study’s analysis of possible mechanisms was exploratory and that the data lacked detailed information about cervical treatment.
Whether vaccination itself reduces the risk of pregnancy complications, and how the observed associations might arise, remains uncertain.
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Key Questions
Did the study prove that HPV vaccination prevents preterm birth?
No. It found an association in observational register data. The design cannot establish that vaccination caused the lower odds or rule out all other explanations.
Which vaccine did the researchers study?
The analysis focused on the quadrivalent HPV vaccine. It excluded births where the first recorded HPV vaccination was bivalent or nonavalent, so the results should not be assumed to apply directly to those vaccines.
Were all pregnancy outcomes lower among vaccinated women?
No. The study reported lower odds for several outcomes, including some forms of preterm birth, but found no significant overall-group differences for prelabor rupture of membranes, stillbirth, or neonatal mortality.
Why did the results weaken after adjusting for cervical lesions?
High-grade cervical lesions were less commonly recorded among vaccinated women, and treatment for such lesions has been linked to pregnancy complications in prior research. The adjustment weakened the associations, but the study lacked detailed treatment data and could not establish a mechanism.
Does this change HPV vaccination recommendations?
The study does not itself establish a change to vaccination guidance. It examined pregnancy outcomes associated with a quadrivalent vaccine; the U.S. schedule cited in the report recommends a nine-valent vaccine series beginning at ages 9 to 12.
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