Background: Vitamin C is a water-soluble antioxidant that not only stimulates and protects collagen synthesis but also plays an important role in maintaining cellular integrity in a normal pregnancy. This study surveyed the effects of ascorbic acid on the serum level of unconjugated estriol and the relationship between unconjugated estriol and preterm premature rupture of membrane (PPROM).Methods: This double-blind, randomized clinical trial recruited 60 patients with predisposing factors to PPROM. The women were randomly divided into two groups of intervention and control and received vitamin C and placebo, respectively. The intervention group received 250 mg vitamin C twice a day and the controls received the placebo only. Unconjugated estriol was measured using the ELISA. All data were extracted and recorded in a checklist and compared using descriptive statistics as well as the x2, Fisher exact, and t tests.Results: The demographic data showed no difference between the two groups. The mean level of serum unconjugated estriol was significantly lower in the intervention group than in the control group (P=0.044). Also, the frequency of PPROM was lower in the intervention group, but the difference was not significant (P>0.05). Unconjugated estriol levels were not significantly different between the healthy women and the PPROM patients. Conclusion: This study demonstrated that vitamin C administration decreased unconjugated estriol levels in the patients with PPROM. The findings of this study also indicated that administration of ascorbic acid was a safe and effective method to reduce the incidence of PPROM. Alteration in unconjugated estriol is an active mediator for this effect.Trial Registration Number: IRCT201012083580N3