Mid-trimester microbial invasion of the amniotic cavity and the risk of preterm birth

To evaluate the rate of mid-trimester microbial invasion of the amniotic cavity (MIAC) in asymptomatic women and its association with preterm birth.

This is a prospective cohort study of asymptomatic women undergoing mid-trimester amniocentesis for genetic testing between 14 and 24 weeks of gestation. For each participant, a sample of amniotic fluid was incubated in an aerobic and anaerobic facultative culture media and another sample was tested for the presence of specific Mycoplasma species (Ureaplasma urealyticum, Ureaplasma parvum, and Mycoplasma hominis) using quantitative-PCR. Results were not revealed to the participants or their health care providers. All participants were followed until delivery. MIAC was defined by a positive culture or a positive PCR for Mycoplasma species. The primary outcome was a spontaneous preterm birth or preterm premature rupture of membranes before 35 weeks of gestation.

We included 812 women at a median gestational age of 16 5/7 (interquartile: 15 6/7-17 4/7) weeks. Twenty-six (3.2%) had a spontaneous delivery before 35 weeks. We observed no case of positive PCR for Mycoplasma species and 4 (0.5%) cases of positive culture that were all considered to be skin contaminants. None of those four cases was associated with preterm birth. Nulliparity, low family income and history of preterm birth were associated with spontaneous delivery before 35 weeks.

We found no case of mid-trimester MIAC using a combination of culture and Mycoplasma-specific PCR techniques in a large cohort of low-risk asymptomatic pregnant women. We estimate that mid-trimester MIAC is rare in low-risk population but more sensitive and broad-range microbiologic techniques, such as 16S DNA detection by PCR, could be further evaluated.