Did smallpox cause stillbirths? Maternal smallpox infection, vaccination and stillbirths in Sweden, 1780-1839

Woods (2009) argued that smallpox was an important cause of stillbirths in the past. While there is strong evidence that maternal smallpox infection could lead to fetal loss, it is not clear whether smallpox infections were a demographically important source of stillbirths. In this paper, we use parish-level data from the Swedish Tabellverket dataset from 1780 to 1839 to test the effect of smallpox on stillbirths quantitatively. We use two empirical strategies: dynamic panel regressions that test the instantaneous effect of smallpox epidemics on stillbirths; and a continuous treatment difference-in-difference strategy to test whether the reduction in smallpox prevalence following vaccination led to a larger decrease in the stillbirth rate in parishes where smallpox was more prevalent before vaccination. We find very little evidence that smallpox infection was a major cause of stillbirths in history. Our coefficients are largely insignificant and close to zero. This is because the vast majority of women contracted smallpox as children and therefore were no longer susceptible during pregnancy. We do find a small, statistically significant effect of smallpox on stillbirths from 1820-39 when waning immunity from vaccination put a greater share of pregnant women at risk of contracting smallpox. However, the reduced prevalence of smallpox limited the demographic impact. Thus, smallpox was not an important driver in historical stillbirth trends and did not contribute to in utero scarring effects for cohorts born when smallpox prevalence was high.