Smallpox is remembered as one of history’s deadliest diseases, but could it also have caused widespread stillbirths? Infectious diseases like smallpox are known to cause severe complications during pregnancy, including fetal death. Yet, how significant was smallpox in driving historical stillbirth rates? In a new study, we used Swedish data from 1780 to 1839 to explore whether smallpox was a major contributor to stillbirths, and the findings are both surprising and illuminating (Schneider et al. 2023).
Smallpox, Pregnancy, and Stillbirths: Setting the Scene
Before the introduction of smallpox vaccination in 1802, smallpox was a frequent and deadly presence in Sweden, with regular epidemics sweeping through communities. During the same period, stillbirth rates were also high, prompting questions about whether the two were linked. Smallpox could be passed from a pregnant mother to her fetus, often resulting in miscarriage or stillbirth, but until now, no one had quantitatively examined just how common this was before and after smallpox vaccination was introduced.
Using detailed parish-level records from Sweden’s Tabellverket dataset, our study aimed to understand whether smallpox played a significant role in stillbirths and how this relationship changed over time. The dataset provided an opportunity to track stillbirth rates alongside smallpox deaths across six decades, offering fresh insights into the potential link between the two.
Key Findings
1. Smallpox Was Not a Major Cause of Stillbirths Before 1820
One of the most striking findings from our study is that smallpox had little effect on stillbirths in Sweden before 1820. Why? Because nearly all women contracted smallpox in childhood and smallpox infection confers lifelong immunity, women were immune to smallpox before they could become pregnant. This early-life immunity meant that even though smallpox was widespread, it did not affect many pregnant women, and as a result, it didn’t significantly contribute to stillbirth rates.
2. Vaccination Changed the Equation
After the introduction of the smallpox vaccine in 1802, Sweden saw a dramatic decrease in smallpox prevalence. However, this also meant that fewer people contracted smallpox in childhood, and women who had been vaccinated as children began to lose their immunity over time. By the 1820s, waning immunity from vaccination left a larger portion of women susceptible to smallpox during pregnancy, leading to a small but statistically significant increase in stillbirths as a result of smallpox. Despite this, the overall prevalence of smallpox was much lower than before, which limited its demographic impact
3. Stillbirth Trends Were Driven by Other Factors
While smallpox was present and could cause stillbirths in pregnant women, our study shows that its role in shaping historical stillbirth trends was limited. The rise in stillbirths observed in the early 19th century is likely attributable to other causes, such as changes in maternal health, obstetric practices, or other diseases. Smallpox, while dangerous, simply did not affect enough pregnancies to be a major factor.

What Does This Mean for the Historical Record?
Our findings challenge previous assumptions that smallpox was a significant cause of stillbirths in history (Woods 2009). While it certainly posed a risk to pregnant women, the impact was smaller than expected, particularly before 1820. This suggests that other factors—perhaps obstetric care or maternal health—were more important drivers of stillbirth rates over time.
Although our study focused on Sweden, the findings have relevance for other regions and time periods. In places where smallpox was endemic and most individuals were exposed as children, the disease likely had little effect on stillbirths. However, in regions where smallpox epidemics hit populations with no prior immunity—such as indigenous populations in the Americas during the Columbian Exchange (1520-1650)—the disease may have had a far more devastating impact on stillbirth rates, contributing to population losses during these epidemics.
This study sheds new light on how smallpox and other infectious diseases impacted fetal health in history, emphasizes the counterintuitive effects that the introduction of vaccination could have, and highlights the need for further research into the complex factors that shaped birth outcomes in the past.
References
Schneider, E. B., Edvinsson, S., & Ogasawara, K. (2023). Did smallpox cause stillbirths? Maternal smallpox infection, vaccination, and stillbirths in Sweden, 1780–1839. Population Studies, (ahead-of-print), 1–16. doi: 10.1080/00324728.2023.2174266
Woods, R. (2009). Death Before Birth: Fetal Health and Mortality in Historical Perspective. Oxford: Oxford University Press.