Providing a detailed medical history when donating blood could be more important than we know - and not just when it comes to screening for disease.
New research has found that men have a far higher mortality rate after receiving blood transfusions using the donor blood of women who have been pregnant. But no one knows why.
Men who received donor blood from women who had been pregnant had a significantly higher mortality rate compared to those who received blood from other men, or women who had never been pregnant, the study has found.
For men under 50, that rate was 1.5 times more in the three years following the transfusion - and the most likely cause of death was an adverse immune response called transfusion-related acute lung injury, an increased risk of which has been previously associated with female donors.
The study, funded by the Dutch Ministry of Health, Welfare and Sports, included 31,118 patients who had between them received 59,320 transfusions of red blood cells in the Netherlands between 2005 and 2015.
The donors were in three categories: male donors, female donors who had never been pregnant (never-pregnant), and female donors who had been pregnant (ever-pregnant). They made up 88 percent, 6 percent and 6 percent of the donor pool respectively.
The patient mortality rate was 3,939 people - 13 percent. And of that number, statistically the largest group was men who had received blood from ever-pregnant donors.
"Among patients who received red blood cell transfusions, receipt of a transfusion from an ever-pregnant female donor, compared with a male donor, was associated with increased all-cause mortality among male recipients but not among female recipients," the researchers wrote in the study.
"Transfusions from never-pregnant female donors were not associated with increased mortality among male or female recipients."
The mortality rate for men receiving red blood from ever-pregnant donors was 101 deaths per 1,000 person-years, compared to 80 deaths for male donors and 78 deaths for never-pregnant donors.
While the study shows a correlation, it doesn't explain why blood from previously pregnant women would have such an effect. The researchers do hypothesise that there could be a "possible mechanism based on immunologic changes occurring during pregnancy".
In an accompanying editorial, two doctors from the Karolinska Institute in Sweden, who were not involved in the study, flag that the next important step will be to look into the potential mechanisms.
"These results are provocative and may - if true - have significant clinical implications. However, other studies have reported conflicting results, and the methodology of these types of studies is complex," they write.
"Therefore, alternative explanations for the observed associations should be considered."
It's also worth nothing that even though the study included a large group of people and a 10-year timeframe, it is limited by the fact the scientists worked with retrospective data.
A study released earlier this year that included 968,264 transfusion recipients did not find the same correlation - but then, it did not analyse the results by donor sex.
Both the researchers and the editorial authors agree that there's more work to be done.
"Further research is needed to replicate these findings, determine their clinical significance, and identify the underlying mechanism," the researchers wrote in their paper.
The study has been published in the Journal of the American Medical Association.