Traditionally, monochorionicity in multiple pregnancies is associated with monozygocity. We present a case of a spontaneous, monochorionic dizygotic, sex-discordant twin pregnancy. The diagnosis of monochorionicity was initially done during first-trimester ultrasound evaluation and then confirmed by postnatal placental pathology. Furthermore, both twins were found to have blood chimerism. We also review the literature on dizygotic-monochorionic twins and blood-chimerism. We suggest that further prospective postnatal genetic studies are needed to define the reliability of prenatal diagnosis of identical twins in cases of monochorionicity.