Antibiotics and high-risk pregnancies

A recent study shows that some antibiotics could lower the risk of pre-term birth in women with high-risk pregnancies but that some others might increase it. The study appeared in the January edition of the Journal of Obstetrics and Gynecology Canada. That infections predispose to pre-term deliveries is not in doubt, almost 50% of such premature births actually linked to infections, which if untreated could put the lives of both mother and child in jeopardy.


Yet, treating infections in the second trimester of pregnancy has been contentious with regard its potential to increase the chances of pre-term births. The findings in this study would no doubt help clear up some of the controversies. That the researchers found that the use in the second-trimester of macrolides and clindamycin significantly reduced the risk of preterm birth relative to placebo, but that metronidazole, called Flagyl or other trade names increased significantly the risk of preterm delivery versus placebo is doubtless, instructive regarding the management of women with high-risk pregnancies. 

High-risk pregnancies could result in delivery before 37 weeks’ gestation. Pregnant women who have infections, for examples bacterial vaginosis, or have positive vaginal fetal fibronectin, a history of preterm delivery and low maternal weight (<50 kg) before they became pregnant, among others, fall into this high-risk category. This study also points to the potential differences in the mechanisms of action of the antibiotics being important in whether they increase or decrease pre-term birth rates. It is possible for example that by essentially killing bacteria, as Flagyl does, hence termed, bactericidal, it releases inflammatory substances toxic to the fetus.

Clindamycin and macrolides ( for example, erythromycin), on the other hand being bacteriostatic, which is to say that they stop the infections but do not kill the bacteria, hence do not release inflammatory toxins, although it is uncertain that this is the explanation for the differences in the actions of the antibiotics observed. For example, these latter antibiotics in particular are more effective than Flagyl in treating mycoplasma infections, which are more commonly associated with preterm births than other bacteria are.

As research efforts continue into understanding better these processes, we could rest assured that women with high-risk pregnancies would be safer as would their babies. Considering the relatively high prevalence of bacterial vaginosis and its association with an increased risk for preterm birth, we would not only be able to help address both issues but also, by not giving them the wrong antibiotic in the process, we would not actually worsen the problems.