Describing their findings as a "little shocking," researchers at the Stanford University School of Medicine say amniotic fluid infections occur at a much higher rate than previously imagined and these same infections seem to be a trigger that causes premature births. Until now, the amniotic fluid surrounding the fetus during gestation has been thought to be generally sterile and safe, free from any risks associated with the health of either mother or child.

The Stanford research team used both standardized culturing techniques and a more precise technique known as PCR, or polymerase chain reaction, to identify microbial infection in amniotic fluid samples of 166 women experiencing premature labor from October 1998 to December 2002. All women in the study were being treated at the Detroit Medical Center, where 113 of them delivered children prematurely.

Culturing involves placing tissue samples, in this case the anmiotic fluid, in specially prepared laboratory dishes and waiting to see which species of bacteria and fungi colonize. This process is rather slow and only a small number of known microbes can be successfully grown in a laboratory setting.

PCR uses high-tech tools to track portions of DNA code within an individual cell. The targeted component, ribosomal RNA, is a part of the cellular structure of all cells of all living organisms. Minute variations of ribosomal RNA can then be used to identify the individual microbe species in question.

According to the research team, led by David Relman, MD, almost 12% of all births in the United States are premature and the rate of premature births to full-term births is on the rise. The Stanford study revealed a link between microbial infection of the amniotic fluid and premature births. The health of the newborn was also found to be affected by microbial infection, with children suffering most where infection was most severe.

The PCR analysis found bacterial or fungal infection in 15% of the study participants’ amniotic fluid although they had expected to find only about half that rate of infection. Cultures using the same fluid samples as those in the PCR analysis revealed infection in only about 10% of the study participants, a situation that highlights the need for more effective screening as early into the pregnancy as possible.

Of the 166 samples tested via PCR, 25 of them were infected by either a fungus or bacteria. Of the samples testing positive for infection, 17 species of bacteria and one species of fungus were identified. One species isolated in PCR analysis is thought to be a newly discovered species althogether. Identifying infection using the lab cultures alone revealed only 11 species of microbes in the tissue samples.

Every woman in the study who tested positive for infection, using either testing method, delivered her child prematurely. Every woman who tested positive using both testing methods delivered her child within 24 hours after her amniotic fluid was collected for analysis. Of the women testing positive for infection by either one of the two testing methods, 68% of them delivered their children within one day of fluid collection.

The infection rate in women delivering before the 25th week of gestation was 27%. In every case, culture samples failed to identify infection but PCR identified the presence of microbes in these cases. When children are born before the 25th week, their chance of survival is only about 30%, a situation that highlights the importance of early diagnosis of infection in amniotic fluid.

The presence of microbial infestation of the amniotic fluid is thought to generate a woman’s natural defenses against inflammation, a defense mechanism that may eventually affect the health of the pregnancy and of the child being carried. Most microbes enter the amniotic sac by way of the vagina but others are carried through the mother’s bloodstream from other parts of her body. Previous studies have shown that women suffering from gum disease or bacterial vaginosis are at greater risk of preterm deliveries than women free of these medical conditions.

Relman writes that, while the microbes we share our bodies with are usually beneficial to our health, things do go awry. When this happens, the sooner we can resolve the infection, the better the health of the patient. In the case of infection of the amniotic fluid, the greater degree of infection, the higher the risk of delivering a very ill child and experiencing childbirth so premature as to be particularly dangerous to the child.

Full details of the Relman study are expected in the August 26 issue of PLoS ONE.