22: Midtrimester microbial invasion of the amniotic cavity and the risk of very preterm birth

To evaluate the prevalence of microbial invasion of the amniotic cavity (MIAC) in the second trimester of pregnancy and its association with very preterm birth. A case-control study nested in a prospective cohort study was conducted in women undergoing mid-trimester amniocentesis for fetal karyotyping between 14 and 24 weeks. Amniotic fluid samples were collected and stored at -80 degree Celsius and all participants were followed until delivery. Each woman who had a spontaneous preterm birth before 35 weeks’ gestation was matched with two women who delivered at term without pregnancy complications. Genomic DNA was extracted and specific quantitative PCR was used to evaluate the presence of intra-amniotic Mycoplasma species, Fusobacterium sp., Gardnerella vaginalis, Porphyromnas gingivalis, Bacteroides sp., Group B Streptococcus and Lactobacillus sp. Broad-range (16S rDNA) PCR was also performed. 3438 women were recruited over a 4-year period including 52 (1.5%) women who had a spontaneous preterm birth before 35 weeks. We observed 5 (9.6%) cases of midtrimester MIAC in our cases and 8 (7.7%) in the control group (p=0.68) using specific PCR. However, we observed a strong (p<0.001) correlation between the bacterial inoculum (using PCR cycle threshold - Ct) and the risk of spontaneous preterm birth (Figure). All three cases with a high bacterial inoculum (Ct<33) delivered before 35 weeks. Using 16S PCR, we observed additional bacteria that affected 21.2% of our cases and 25% of our controls. Midtrimester MIAC is commonly found in low-risk population and is not necessarily associated with preterm birth. Our findings suggest that the amniotic cavity is not completely sterile in normal conditions. However the risk of very preterm birth increases significantly with the level of midtrimester amniotic bacterial inoculum.