Infections as a Cause of Preterm Birth: Amniotic Fluid Sludge—An Ultrasound Marker for Intra-Amniotic Infections and a Risk Factor for Preterm Birth

Preterm labour (PTL) affects about 11% of all deliveries world-wide. It is a major cause of perinatal morbidity and mortality. Although the precise cause is unknown in about 50% of cases, infections are thought to be a major contributing factor. These infections are more common in earlier preterm deliveries. While some women with these infections will manifest the classical features of fever, tachycardia (maternal and/or fetal), leucocytosis, raised biomarkers of infections, and uterine tenderness/irritation, others will be asymptomatic. Some of the women may develop a short/dilating cervix without any obvious contractions. Identifying such women is potentially challenging. Evidence has shown that a condensation of echogenic particles just above the cervix—amniotic fluid (AF) sludge, identified by ultrasound—is a marker for microbial invasion of the amniotic cavity (MIAC) and preterm birth (PTB) in both asymptomatic and symptomatic women (including those with a short or normal cervix). Those with a short cervix with AF sludge have a significantly greater risk of progression to PTB. Treatment with antibiotics has been shown in some but not all case series to result in a resolution of the sludge and either a delay or prevention of PTB. The widely varied results from treatment could be related to the antibiotics used and the route of administration. The use of the parenteral combination of clindamycin, a cephalosporin, and metronidazole has been shown to be more effective compared to azithromycin. Here we review the literature on the relationship between the sludge and PTB and conclude (1) that the AF sludge is an ultrasound marker of MIAC and PTL and (2) that following its diagnosis, appropriate counselling should be offered and the triple antibiotic combination offered. We suggest that randomised trials should be undertaken to determine the most efficacious antibiotic combination.