page_title: Monochorionic twins
section_title: Amniocity and zygosity
hierarchical_section_title: Monochorionic twins / Amniocity and zygosity
caption_attribution_description: English: Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right.
context_page_description: Monochorionic twins are monozygotic twins that share the same placenta. If the placenta is shared by more than two twins, these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. Seventy-five percent of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic. If the placenta divides, this takes place before the third day after fertilization.
context_section_description: 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 the ninth day after fertilization. Monoamniotic twins are always monozygotic (identical twins). Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.
caption: Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right.
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