Getting a flu shot was a must on my pregnancy to-do list because I knew it could protect my babies—and my ob-gyn emphasized its importance to boot.

About half of pregnant women, like me, typically get the flu shot—but the category includes people who are some of the most vulnerable to actually coming down with influenza. If an expecting mom gets the flu, her symptoms are often much worse, and she runs a higher risk of premature delivery. That much has long been known.

But the real reason for why that is may have just been discovered: As Time reports, a new study out of Stanford University published in the Proceedings of the National Academy of Sciences found that certain cells collected from pregnant women reacted too strongly when attempting to fight off the infection.

That might sound like a positive thing, but the side effects of the overkill response can be damaging. For instance, too many cells in the lung trying to solve the problem can result in inflammation which makes it hard to breathe, according to the study.

It's a surprising finding because it's been commonly understood previously that flu hits pregnant women harder because their immune systems are compromised as a means of keeping the body from attacking the growing fetus.

The study's authors, not surprisingly, said they urged all pregnant women to get those flu shots.

And they're not the only ones, of course. The American College of Obstetricians and Gynecologists recently announced new guidelines that indicate the importance of a flu shot while pregnant. And the National Foundation for Infectious Diseases just held a press conference urging everyone older than six months of age to get a vaccine this flu season.

Will you?

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