Expectant mothers go through changes in their organs and immune system that leave them open to severe flu infections.

Now an international study shows that flu shots can help keep them out of the hospital.

Roughly one-half of pregnant women in recent years reported receiving a flu shot.

But research in the journal Clinical Infectious Diseases reports that flu shots could lower their risk of hospitalization by about 40 percent.

Based on medical records of 2 million pregnant women, researchers found the vaccine helped regardless of trimester and also protected those with other conditions, like asthma and diabetes.

During the study, 80 percent of subjects were pregnant for part of a flu season, which suggests that future mothers will likely encounter the flu while expecting.

The Centers for Disease Control and Prevention, which co-authored the study, says flu shots have a good safety record with pregnant women, but added that they should not get the nasal spray vaccine.