August 26 (HealthDay News) -- Infections of amniotic fluid are more common than previously believed and may be a major cause of premature births, says a study led by researchers at Stanford University School of Medicine.

About 12 percent of all births in the United States are premature, and the frequency of premature birth is increasing.

The Stanford researchers analyzed amniotic fluid from 166 women in preterm labor (113 went on to deliver their infants prematurely) and found that 15 percent of the samples contained bacteria or fungi, compared to previous estimates of 10 percent. Some of the bacteria and fungi found in the samples hadn't been suspected of playing a role in preterm delivery.

Women with the highest degree of amniotic fluid infection were most likely to deliver more premature, sicker infants, the researchers said.

"To find that this amniotic compartment, which we have traditionally viewed as somewhat sacrosanct, is infected significantly more often than we thought is a little shocking," senior author Dr. David Relman, a professor of infectious disease and of microbiology and immunology, said in a Stanford news release.

The study was published in the Aug. 26 issue of the journal PLoS-ONE.

"If we could prevent these infections in the first place, or detect them sooner, we might one day be able to prevent some of these premature births," study first author Dr. Dan DiGiulio said in the news release.

The researchers are now conducting a larger study of amniotic fluid collected through routine amniocenteses at about the 20th week of pregnancy to determine if infections can be detected before the onset of preterm labor. The findings may help lead to new methods of prevention or treatment.

SOURCE: Stanford University, news release, Aug. 26, 2008