Dichorial Or Dichorionic Twins
Dichorial or dichorionic twins have separate chorionic membranes (envelops the embryo). This is often the case of fraternal twins which have separate chorions (the outermost membrane of amniotic sacs surrounding the fetus) as well as separate placentas (source of oxygen and nutrients). Since fraternal twins develop from two independent zygotes, they are always dichorionic. However, some identical twins which come from the same zygote (monozygotic twins) may also be dichorionic; this is generally influenced by the time of the zygote splitting. Around 30% of monozygotic twins which have split a few days after fertilization result to two separate blastocysts; this makes them implant independently and become dichorial.