Cervical dilatation thresholds for initiation of group B streptococcus antibiotic prophylaxis for women with spontaneous preterm labor.

OBJECTIVE

To determine the optimal time for initiating group B streptococcus (GBS) antibiotic prophylaxis for women in spontaneous preterm labor.

METHODS

In total, 227 women delivering singleton infants after presenting with spontaneous preterm labor and intact membranes at 24 0/7-36 6/7 weeks were evaluated, as well as 150 undelivered women with threatened preterm labor during the same time period. The date and time of each cervical examination throughout labor were recorded. We calculated the percentages who would have correctly received at least 4 h of GBS prophylaxis if antibiotics were routinely initiated for various cervical dilatation thresholds during labor, as well as the percentage of undelivered women who would have received unnecessary antibiotic exposure at each cervical dilatation cutoff.

RESULTS

Delaying antibiotics until cervical dilatation reached 2 cm or greater would have resulted in 62.1% receiving four or more hours of antibiotics, compared to 66.5% if antibiotics were started on all women at admission (p = 0.33), while significantly reducing unnecessary antibiotic exposure in undelivered women from 100% to 62.0% (p < 0.001). The 2-cm threshold was applicable regardless of gestational age period or prior vaginal delivery ≥ 20 weeks.

CONCLUSIONS

GBS antibiotic prophylaxis may reasonably be withheld for women with suspected preterm labor until the cervix reaches 2 cm or greater at any time during labor.