Craniopagus twins the set of twins as being a unique malformation and a punishment from their mother’s mistake. Furthermore, in Ambroise Pare’s book, "On Monsters and Marvels", various types of "supernatural" twinning are illustrated and described as "monstrous and marvelous creatures that proceed from the judgment of God". This published history suggests that conjoined twins, and in specific craniopagus twins, were viewed as literal monsters during that era. Craniopagus twins Craniopagus twins are conjoined twins that are fused at the cranium. This condition occurs in about 10–20 babies in every million births in the United States. Among this small group, cephalic conjoining,
Monochorionic twins Monochorionic twins Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. 75% of monozygotic twin pregnancies are monochorionic; the remaining 25% are "dichorionic diamniotic". If the placenta divides, this takes place after the third day after fertilization. Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), but sometimes, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after