Women have a very strong immune response during pregnancy when it comes to fighting off influenza, according to new research.

This unexpected finding might explain why pregnant women seem to get more sick from the flu compared to healthy, non-pregnant adults. The findings of the study are surprising, as it was previously thought that immune responses in pregnant women were weakened in the presence of influenza viruses in order to prevent the body from rejecting the fetus.

Researchers from the Stanford University School of Medicine and Lucile Packard Children’s Hospital Stanford conducted a study, the first of its kind, to analyze the reaction of immune cells to the influenza virus from pregnant women.

The study involved taking immune cells from 21 pregnant women, and 29 non-pregnant, healthy women who were exposed to a variety of flu viruses in a controlled laboratory environment. The researchers also examined the immune cells from the pregnant women six weeks after having their babies. Responses to a strain of influenza, H3N2 and pandemic H1N1 were studied.

In comparison to the same cells from non-pregnant women, H1N1 caused pregnant women’s natural killer and T cells to produce more cytokines and chemokines, which help draw other immune cells to an infection site.

Senior study author Catherine Blish, MD, PhD, assistant professor of infectious diseases, says she was surprised at what they had discovered.

“We now understand that severe influenza in pregnancy is a hyperinflammatory disease rather than a state of immunodeficiency. This means that treatment of flu in pregnancy might have more to do with modulating the immune response than worrying about viral replication,” said Blish in a statement.

The findings of the study are published online in the Proceedings of the National Academy of Sciences.