Twins are the most frequent form of multiple births. The organization of the fetal membranes depends on the type of twins. In addition, with monozygotic twins, the timing of the zygotic division is decisive.
The most frequent form of twins are the dizygotic ones (7 to 11 in 1000 deliveries), also known as «sibling twins» or «false twins». These twins stem from two oocytes that matured during the same menstruation cycle and were fertilized by two different sperm cells. In other words, genetically, the two zygotes that arose with the fertilization differ just as much as two normal siblings. Thus dizygotic twins can be of the same or opposite sex.
Dizygotic twins implant themselves separately and develop membranes that are independent of each other. Each twin has its own placenta, its own chorion and its own amniotic cavity.
It sometimes happens that the two placentas lie so close to each other that they fuse; the same thing can also occur with the chorions of the two twins. Thus immunologic incompatibility reactions can be observed since the fusion of the two placentas makes an exchange between the circulation systems possible.
The second kind of twins, which stem from a single oocyte, are known as monozygotic twins or «real twins» (the frequency amounts to 3 to 4 in 1000 deliveries). They result from a division of the blastomeres in various stages of their development.
A. If the division takes place during the cleavage (stage 2), e.g., in the 2-cell-blastomere stage, the monozygotic twins implant themselves separately (after the zona pellucida has disappeared), similar to dizygotic twins.
They do not share their membranes: each twin has its own placenta, its own chorion and amnion.
B. In the majority of the cases, the cleavage occurs in the blastocyst stage 3-4. The embryonic bud divides itself in the interior of the same blastocyst cavity into two masses of cells. Both embryos possess the same chorion and the same placenta, but each has its own amnion.
C. Finally, in rare cases, the cleavage can also take place in the stage of the double-layer embryo, directly before the appearance of the primitive streak (stage 5). For both twins, this kind of separation leads to a common placenta, a common chorion and a common amniotic cavity. As a rule, although only one placenta is present, the blood supply for the twins is well equalized. Nevertheless, sometimes the perfusion of one of the fetuses is favored which could partially explain why large size differences occur.