Investigation: The cause of discordance in dichorionic diamniotic (DD) twins is still unknown. The authors aimed to compare decorin (DCN) and oxidative/antioxidative state levels between the placentas of discordant and concordant twins. Materials and Methods: Prospective study of 43 spontaneous DD twin pregnancies included and placentas samples taken from each twin and prepared for homogenization. Total oxidant/antioxidant status levels in placental tissue were determined by automated colorimetric method. Decorin levels were detected by using ELISA method; 23 of these were discordant and 20 of them were concordant. Results: DCN levels in the placentas of the low birth-weight twins were significantly lower than the levels of the placentas of appropriate gestational age twins (p = 0.006). There were no statistically significant differences in total antioxidant status (TAS), total oxidant status (TOS), or arylesterase (ARES) levels in discordant (p = 0.631, p = 0.370, and p = 0.079, respectively) and in the placental DCN, TAS, TOS, or ARES levels of the concordant twins (p = 0.407, p = 0.035, p = 0.194, and p = 0.979, respectively). When the authors compared the twins of similar birth weight, the DCN, TAS, and TOS levels were significantly lower in the discordant twins (p < 0.001, p < 0.001, and p = 0.002, respectively). Conclusions: Decreased levels of DCN in discordant twin fetuses compared to the same birth weight-concordant twins shows that it contributes to disease pathogenesis.