Men and women are biologically different, and that may apply to their blood as well — at least for women who have been pregnant.

In a study published in JAMA, researchers from the Netherlands studied more than 31,000 people who received blood transfusions at six Dutch hospitals from 2005 to 2015. They tracked whether the recipients had received blood from male donors, female donors who had never been pregnant, or female donors who had been pregnant. The researchers then analyzed death rates for three years.

The only group that saw a difference based on the type of donor was men who received blood from women who had been pregnant. Those men were more likely to have died after three years, compared to men who received blood from a male donor or from a woman who had never been pregnant. Women who received blood transfusions did not see a higher risk of death regardless of whether the blood came from a man or a woman. The results held even after the scientists accounted for differences in the severity of diseases that required the transfusions in the first place.

Previous work pointed to one change in women’s blood — especially women who have been pregnant — that may make transfusions risky. Known as transfusion-related acute lung injury (TRALI), it is caused by antibodies that mothers make when they are pregnant, and can cause severe lung problems. But TRALI often occurs soon after a transfusion, and the deaths in the study were recorded for as long as three years.

Read more: 7 Things to Know Before You Donate Blood

There may be other immune-related changes associated with pregnancy that can change mothers’ blood, the researchers say. There may also be changes in iron that could explain some of the poorer outcomes of the pregnant women’s transfusions.

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For now, experts say there isn’t enough data to support changing current blood donation policies, but the findings suggest that more research is needed on the role that gender, and pregnancy, may play in blood transfusions outcomes.

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