Amniotic sac not so sacrosanct

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The clear, slightly yellowish amniotic fluid that envelops unborn babies during pregnancy harbors previously unidentified and unrecognized infection-causing microbes, researchers report online August 26 in PLoS ONE. The study adds evidence to the premise that infectious microbes found in amniotic fluid can cause premature birth.

“We were surprised with the amount of unexpected bacteria we found in the fluid and the fact we encountered new species of bacteria,” says physician Daniel DiGiulio of the Stanford University School of Medicine and lead author of the study.

Screening the amniotic fluid with both conventional methods and a novel DNA sequencing approach, the scientists identified infectious bacteria or fungi in 25 of the 166 women in the study. That prevalence for infection — 15 percent — is 50 percent higher than in past studies, DiGiulio says. The level of infection is likely even higher because the tests do not yet identify all pathogenic material in the fluid, he adds.

“We only know the names of relatively a few of all the bacteria that exist, and a lot of them are difficult to culture or can’t be cultured with our current technology,” comments physician Robert Goldenberg of the Drexel University College of Medicine in Philadelphia. He was not surprised by the results and suspects that as scientists continue to study amniotic fluid with improved techniques many more pathogens will be identified.   

A baby born before 37 weeks is considered premature. In 12 percent of pregnancies in the United States, babies are born prematurely. Early birth is the leading cause of neonatal death worldwide, according to the National Institutes of Health.

In about half of those cases, the trigger of the premature birth remains unknown, DiGiulio says. But doctors suspect that infection-causing microorganisms living in the amniotic fluid probably trigger a response from a woman’s body. The microbes can infiltrate the sack from the vagina or by way of the bloodstream from other parts of the body, including the mouth. As a result, the immune system tries to fight the infection, causing inflammation that can cause contractions and birth of the child.

To better study if infection leads to early birth, DiGiulio and colleagues, including researchers at the Wayne State University School of Medicine in Detroit, studied the amniotic fluid of 166 women who went into preterm labor at the Hutzel Women’s Hospital in Detroit from 1998 to 2002. Of the total, 113 women delivered prematurely and 25 showed infection. All 25 women with infected fluid gave birth prematurely.

Of those women, the ones harboring the highest number of infectious bacteria had their babies the earliest — a telling sign of the link between infection and premature birth, DiGiulio says.

“There’s lots of evidence that inter-uterine infections cause preterm birth, especially early preterm birth,” notes Goldenberg.

But DiGiulio says studies have yet to confirm that infections do in fact cause preterm labor or premature birth. To show definite causality, much larger studies need to be done, he explains. Currently he and his colleagues are studying fresh, rather than stored, amniotic fluid to see if it is possible to identify the infections before they induce preterm labor or premature birth.

“If we can do that,” he says, “we could potentially create a treatment for these infections and prevent a lot or possibly all of premature births.”