Women who received the quadrivalent HPV vaccine before becoming pregnant were less likely to give birth prematurely and had lower rates of several other pregnancy and delivery complications, according to a study from Karolinska Institutet published in The BMJ on 7 October 2026.
The research drew on Swedish national registers and covered 624,713 pregnancies among first-time mothers aged 16 to 35 between 2006 and 2023. Of those pregnancies, 92,620 involved women who had been vaccinated with the quadrivalent HPV vaccine, which guards against four common types of the virus, before conception.
Compared with unvaccinated women, those vaccinated before pregnancy had a consistently lower risk across the complications examined, with the clearest signal for preterm birth. Their likelihood of giving birth early was roughly five percent lower, and their likelihood of giving birth very early was about 15 percent lower, Karolinska Institutet reports.
The researchers also observed that the associations were stronger among women who had been vaccinated before the age of 17. Zhongsong Zhang, a doctoral student in the institute's department of medical epidemiology and biostatistics, said this fits with what is already known about the vaccine.
"We also saw that the associations were stronger among women who had been vaccinated before the age of 17. That is consistent with previous knowledge that the HPV vaccine provides the best protection against HPV-related disease when given at younger ages," Zhang said, according to Karolinska Institutet.
Why it matters
HPV, or human papillomavirus, is the leading cause of cervical cancer. Beyond that well-established link, both HPV infection and treatments for cell changes in the cervix have themselves been tied to a higher risk of certain pregnancy complications. That background makes it worth asking whether vaccination, which prevents the infection in the first place, might also affect pregnancy outcomes.
The study examined several such outcomes: preterm birth, premature rupture of membranes, infants small for gestational age, stillbirth, and death among newborns. According to Karolinska Institutet, vaccinated women showed lower risks across the board, not just for preterm delivery.
The public health arithmetic is part of the significance. Jiayao Lei, an associate professor in the same department at Karolinska Institutet, noted that even a modest reduction in preterm birth risk can matter when multiplied across a population, because HPV vaccination is given to millions of young people worldwide.
"Even a relatively small reduction in the risk of giving birth prematurely can be significant at the population level, since HPV vaccination is given to millions of young people around the world. Our results suggest that the benefits of HPV vaccination may extend beyond cancer prevention and also include complications that can arise during pregnancy and childbirth," Lei said, according to Karolinska Institutet.
What the study can and cannot show
The research is an observational study, which means it can demonstrate an association but cannot establish that vaccination caused the lower risks. The authors accounted for multiple factors that could influence the results, but Karolinska Institutet states that a causal link cannot be confirmed from this design.
What the study does offer is scale and duration. It covers a large share of the Swedish population over a period of nearly two decades, drawing on register data rather than a smaller sample. That breadth strengthens the reliability of the observed patterns, even if it does not settle the question of cause.
The work was carried out with researchers from the University of Gothenburg, Sahlgrenska University Hospital, and the Norwegian Institute of Public Health. Funding came from several sources, including the Swedish Research Council, Forte, the Swedish Cancer Society, Karolinska Institutet, and ALF grants. Information on potential conflicts of interest is available in the scientific article itself.
The paper is titled "Quadrivalent HPV vaccination before pregnancy and pregnancy outcomes: A nationwide, population-based case-control study." Its authors are Zhongsong Zhang, Alexander Ploner, Verena Sengpiel, Nestory Kasheshi, Bo Jacobsson, Joakim Dillner, Pär Sparén, Cecilia Kärrberg, and Jiayao Lei. It was published online in The BMJ on 7 October 2026, with the DOI 10.1136/bmj-2026-100739.
A vaccine whose reach may go further
The findings add a reproductive-health dimension to a vaccine that is usually discussed in the context of cancer prevention. If the associations hold up in further research, they would suggest that the benefits of vaccinating girls before they reach childbearing age could include outcomes that only become visible years later, during pregnancy and delivery.
The age gradient the researchers found — stronger associations among those vaccinated before 17 — points in the same direction as the existing evidence on timing. Protection against HPV-related disease is known to be most effective when the vaccine is given earlier, and this study suggests the pregnancy-related patterns follow a similar shape.
For now, the results are best read as a signal rather than a conclusion. The authors themselves frame the work that way, and the observational design leaves room for factors that were not measured or fully adjusted for. Still, the size of the cohort and the consistency of the findings across several outcomes make the pattern difficult to dismiss as noise.
What to watch
Further studies, ideally using different populations and designs that can better address causation, will be needed to test whether the link is real and how large it is. In the meantime, the study gives researchers and public health officials another reason to look closely at the timing of HPV vaccination relative to pregnancy — and at what else the vaccine may be doing beyond its established role in cancer prevention.
