Pregnant and feeling under the weather? Figuring out if you have a cold or flu is essential, as influenza can blossom into some serious and deadly conditions for a pregnant woman.

With all the other changes your body is now undergoing, the last thing you need is a bout of the flu. Physiological changes due to pregnancy include the immune system and heart and lungs. Influenza infection, with its associated symptoms, can cause a host of pregnancy problems including premature births. So the recommendation to get a flu shot right away, which is completely safe for pregnant women, is a solid suggestion.

This will boost a woman’s immune response to the flu virus and will hopefully stave off the nasty bug. Here are a few important facts to keep in mind.

1. Changes to a pregnant woman’s immune system make her vulnerable to infections and other illnesses. The immune system is normally depressed during pregnancy so that the mother’s body doesn’t reject the growing baby as a foreign body. But a lowered immune response means a woman is at a higher risk for illnesses such as pneumonia, a life-threatening infection.

A recent study by Stanford University has put forth a different theory as to the increased risk of serious illnesses in pregnant women. In the lab, white blood cells, which fight infections in the body, were infected with the influenza virus. These infected cells attracted more white blood cells. These excessive cells irritate the lungs, causing more cells to attack. This mechanism may be responsible for excess fluids in the lungs, leading to pneumonia.

Dr. Catherine Blish, assistant professor of infectious diseases at Stanford School of Medicine and the study’s senior author said, “Having too many immune cells in the lung can cause inflammation that makes it hard to breathe,” she says. “If these findings are confirmed in bigger studies and then natural infection, they could explain why pregnant women do so poorly.” There is something in the influenza virus that brings out this response in immune cells, said Blish.