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Not all MZ twins are identical - the twin-twin transfusion syndrome

Some MZ twins share the one placenta and the outer membrane (chorion) and so are called monochorionic (MC). A few share the inner membrane as well and these monoamniotic twins need careful monitoring during pregnancy. MC twins may in fact be much more different from each other than are DZ twins because of a one-way circulation in the placenta whereby one twin shunts blood to the other. Such twins are very different in birthweight (often 500 g or more) and typically the larger one (the donor) is red, while the smaller one is pale. In fact the larger one is more at risk because the blood is so thick. The size difference may persist throughout school and we have seen 15 year old MZ twins where one is 15cm taller than the other. There is growing evidence that some differences in abilities between the twins may be associated with this condition, called the twin-twintransfusion syndrome, but studies are not yet conclusive

Sometimes this is associated with the common MZ situation of mirror-image twins, where one is right and one left-handed. There can be other aspects of mirror-imaging such as the direction of hair whorls, or on which side of the mouth teeth erupt. So being different in such aspects of laterality does not mean multiples are DZ. It also does not mean the opposite-if twins differ in handedness, you cannot assume they are mirror-image MZ twins.