Estimates of the effect of foetal health shocks may suffer from survivorship bias. The foetal origins literature seemingly agrees that survivorship bias is innocuous in the sense that it induces a bias towards zero. Arguably, however, selective mortality can imply a bias away from zero. In the case of the 1918 flu pandemic, a suppressed immune system may have been protective against the most severe consequences of infection. We use historical birth records from the maternity hospital of Bern, Switzerland, to evaluate this possibility. Our results suggest that a careful consideration of survivorship bias is imperative for the evaluation of the 1918 flu pandemic and other foetal health shocks.