A randomized study of antibiotic therapy in idiopathic preterm labor

A randomized study was undertaken to test the effects of antibiotics as an adjunct to established methods of tocolysis. One hundred fifty patients with cervical dilation of 1 cm or more and no clinically identifiable cause for preterm labor qualified for the study. Fifty-three (35%) received 500 mg ampicillin orally every 6 hours, 50 (33%) received 500 mg erythromycin orally every 6 hours, and 47 served as controls. Antibiotics were prescribed for 10 days, and the treatment was not altered by the results of cervical cultures. Of these 150 patients, 16 (11%) had positive amniotic fluid cultures. The pregnancies with positive amniotic fluid culture were characterized by significantly less time gained after admission (2.6 versus 28.7 days) and lower birth weight (1262 versus 2470 g) than in those with negative cultures. Histologic studies of the placenta revealed chorioamnionitis in 22 (16%) of 134 patients with negative amniotic fluid cultures; these pregnancies were associated with less time gained after admission (12.5 versus 31.9 days) and lower birth weight (1680 versus 2618 g) compared with pregnancies without histologic chorioamnionitis. In patients with similar gestational age and cervical dilation, the adjunctive use of antibiotic therapy resulted in a statistically significant delay from admission to delivery (30 versus 17 days). In cases of negative amniotic fluid cultures, cervical colonization with group B streptococcus and/or Gardnerella vaginalis increased the risk of prematurity, which improved significantly when ampicillin was given.