Twin
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Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic, meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic, meaning that each
Identical twins can be difficult to visually distinguish especially when young, as demonstrated by brothers Billy and Bobby Mauch.
Adult fraternal twins
Fraternal twin brothers as young babies
Austrian TV host Mirjam Weichselbraun (right) and her fraternal twin Melanie look very similar but with a significant difference in height.
Monoamniotic twins
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Monoamniotic twins are identical or semi-identical twins that share the same amniotic sac within their mother's uterus. Monoamniotic twins are always monochorionic and are usually termed Monoamniotic-
Twin
…where the blastocysts have fused. Monochorionic twins share the same placenta, and thus have a risk of twin-to-twin transfusion syndrome. Days 4–8 Monochorionic–monoamniotic Monochorionic twins share the same amnion in 1–2% of monozygotic twin pregnancies. Monoamniotic twins are always monozygotic. The survival rate for…
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.