Objective: To evaluate whether 400 milligrams of vaginal progesterone, as a maintenance tocolysis, could prolong the latency to delivery, prevent preterm birth, reduce adverse neonatal outcomes, and delay cervix shortening after arrested preterm labor.

Materials and Methods: An open-label randomized controlled trial was conducted on 80 singleton pregnant women with symptomatic preterm labor, who were successfully arrested with Nifedipine as acute tocolysis, at gestational ages 24 to 33 weeks and 6 days, at Hatyai Hospital. Participantswere then randomly allocated using a permuted block of four to either the vaginal progesterone group or the control group.

Results: The vaginal progesterone group resulted in a longer mean latency to delivery at 47.3±17.1 versus 34.2±18.1 days (p<0.001), a lower preterm birth rate at 12.5% versus 35.0% (p=0.036), a higher gestational age at delivery of 38.5±1.3 versus 36.7±2.5 weeks (p<0.001), and improved birth weight of 2,932.6±309.9 versus 2,627.1±517.3 grams (p=0.002). There were no significant differences in the delay of cervical shortening at –3.2±2.8 versus –4.7±5.0 mm (p=0.192) or the rate of neonatal intensive care unit (NICU) admission of 4% versus 11% (p=0.086) or the rate of birth asphyxia of 5% versus 10% (p=0.675).

Conclusion: Administering 400 milligrams of vaginal progesterone as a maintenance tocolysis in arrested preterm labor effectively prolongs latency to delivery, reduces the rate of preterm birth, and improves birth weight. However, it fails to delay cervical shortening and does not decrease the rate of NICU admission or occurrence of birth asphyxia.