The reason this study was done because there has been a very well replicated association between the plasma (not red cells) donated from previously pregnant women and the risk of TRALI, a very rare but deadly transfusion reaction. After this association was made clear, blood banks no longer collect plasma from previously pregnant ("ever pregnant") women, and the overall rate of TRALI has dramatically dropped (from about 35 fatalities in the US per year to ~15).
Source: MD/lab medicine resident