Gemelos monocigóticos dicoriónicos-diamnióticos: entendiendo su fisiopatogenia gracias a las técnicas de reproducción asistida

Twinning has fascinated human beings over the centuries. It is well recognized that this condition is increased after assisted reproductive techniques and several factors have been identified that appear to predispose patients to this phenomenon. The majority of twins that occur with assisted reproductive techniques are dizygotic and are a direct result of a multi-embryo transfer. Monozygotic twins, however, appear to also be increased. Traditional dogma suggests that if division of the embryo takes place during the cleavage stage (day 1-3), the monozygotic twins implant themselves separately (after the zona pellucida has disappeared), similar to dizygotic twins. Each twin thus has its own placenta, its own chorion and amnion. In the majority of the cases, the division occurs in the blastocyst stage (day 4-8). In these cases, 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. If this process occurs afterwards (day 8-12) monochorionic-monoamniotic twins develop. Theoretically, the day the embryo splits will regulate the chorionicity and amnionicity of the pregnancy. Our series of cases, however, do not concur with this dogma. We observed late embryonic splits that resulted in monozygotic pregnancies with two chorions and two amnions.