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Getting blood from a woman who has ever been pregnant could be risky for men, a new study from the Netherlands suggests. The study, published today (Oct. 17) in the journal JAMA, found that men who received blood transfusions from previously pregnant female donors were 13 percent more likely to die during the study period, compared with men who received blood transfusions from male donors. In contrast, men who received blood transfusions from women who had never been pregnant were not at increased risk of death over the study period, compared with men who received transfusions from other men, the study found. And women who received blood from women either with or without a history of pregnancy were not at increased risk of death, compared with women who received blood from male donors. The findings "are provocative and may — if true — have significant clinical implications," as well as affect the blood donation process and the use of blood in transfusions, Dr. Ritchard Cable, of the American Red Cross Blood Services, and Dr. Gustaf Edgren, of the Department of Hematology at Karolinska University Hospital in Stockholm, wrote in an editorial accompanying the study. However, the results are preliminary, and it's possible that there are alternative explanations for the findings. Therefore, more studies are needed to confirm the results, Cable and Edgren said. For now, the criteria for donating blood should not change, they said. It's not exactly clear why blood from female donors who have been pregnant is linked with an increased risk of death among male recipients. However, the researchers speculated that changes that occur in a woman's immune system during pregnancy could play a role. Previous studies suggested that men who receive blood transfusions from women are at higher risk of death than men who receive transfusions from male donors. But it wasn't clear if a history of pregnancy among female donors affected this link. In the new study, researchers from Leiden University Medical Center analyzed information from more than 31,000 individuals who received red blood cell transfusions in the Netherlands between 2005 and 2015. People were included in the study only if they received transfusions exclusively from one of three types of donors: male donors, female donors with a history of pregnancy, and female donors without a history of pregnancy. (This means, for example, that patients could not be included in the study if they received blood transfusions from both a male donor and a female donor with a history of pregnancy.) The participants were followed, on average, for a little over a year after their transfusions. Overall, nearly 4,000 participants died during the study period. For male patients, there were 101 deaths per 1,000 people per year among those who received blood from female donors with a history of pregnancy, compared with just 80 deaths per 1,000 people per year among those who received blood from male donors. This increased rate of death was seen only for men ages 50 and younger. Among the men who received blood from women without a history of pregnancy, there were 78 deaths per 1,000 people per year — about the same as the rate of death among men who received transfusions from male donors. For women, there was not an increase in the rate of death among those who received blood from ever-pregnant or never-pregnant women, compared with those who received blood from male donors. Doctors have known that, in rare cases, people who receive a blood transfusion develop a condition called transfusion-related acute lung injury (TRALI), a serious inflammatory reaction in the lungs that can result in death. Antibodies or other immune system factors that women develop in pregnancy could trigger TRALI in male blood-transfusion recipients, the researchers hypothesized. But the study had limitations. For example, because the patients in the study received blood transfusions from only one type of donor, these patients tended to receive fewer transfusions than the average transfusion patient. (The chances that a patient received transfusions from more than one type of donor increases with the number of transfusions.) So it's unclear how well the findings apply to the general population of transfusion patients (who may be sicker than those in the study), the researchers said. In addition, the finding of an increased risk of death among men who received transfusions from ever-pregnant women was true only for men ages 50 and younger. "This makes the findings very tentative, and they require validation in other studies," the researchers wrote. But if future studies do show a similar link, "blood centers and transfusion services will need to mitigate this risk," Cable and Edgren said in their editorial. This might be done by matching donors and recipients based on sex, or by modifying donor blood in such a way as to further remove immune system factors that might be responsible for the link, they said.