- To determine the relationship between maternal serum ferritin concentrations in the second trimester and the risk of preterm delivery (PTD). A prospective observational study was conducted. Fifty consecutive women with singleton pregnancies, who were admitted to the Maternal Fetal Medicine Unit due to preterm labor in the second trimester, were included. Maternal serum samples for determination of ferritin concentrations were obtained. Multiple logistic regression analysis was performed to control for confounders. Out of fifty patients enrolled in the study, 38% (19/50) delivered prematurely. Eight women (16%) had maternal serum ferritin concentrations above 30 ng/ml in the second trimester. Among them, 75% (n = 6) subsequently presented with preterm delivery (odds ratio (OR) = 6.7 with 95% confidence interval (CI) 1.1-56.2, p = 0.04). Only two patients with increased maternal ferritin concentrations delivered at term. However, 13 patients with second trimester ferritin concentrations below 30 ng/ml had preterm delivery. No significant differences in mean maternal ferritin concentrations were found between patients who delivered preterm and those that delivered at term, 31.9 +/- 50.6 vs. 13.6 +/- 15.2, respectively (p = 0.064). Using a multivariable analysis, controlling for anemia, leucocytosis and maternal age, increased serum ferritin concentrations were found to be an independent risk factor for PTD (OR = 8.6; 95% CI 1.4-52.5; p < 0.019). No significant correlation was found between serum ferritin concentrations and gestational age at birth (Pearson correlation coefficient r = -0.093; p = 0.522). Maternal ferritin concentrations above 30 ng/ml in the second trimester can serve as a marker for preterm delivery. However, since no correlation was found between serum ferritin concentrations and gestational age at birth, the routine use of serum ferritin as a marker for preterm delivery warrants further investigation.