"These are really upsetting numbers," Dr. Anne Schuchat, director of the U.S. Centers for Disease Control and Prevention's Center for Immunization and Respiratory Diseases, said during an afternoon press conference.

"We have obstetricians here at CDC who are coordinating the outreach as well as the surveillance efforts around it," she said. "And they're talking to doctors around the country who have never seen this kind of thing before. We don't track seasonal flu. We haven't in the past tracked seasonal flu complications in pregnancy. But what we are seeing is quite striking."

Schuchat said it's not yet clear whether there is something different about the H1N1 flu's effect on pregnant women, or whether researchers are noticing its effect on pregnant women more because the virus is being monitored closely.