A question which is commonly encountered by families with twins is “are they identical or fraternal?”

Knowing whether twins are identical or fraternal is often difficult. Appearance alone often is not a reliable method in determining twin type because identical twins do not always look exactly the same, and fraternal twins can sometimes look very similar, leading to confusion about exactly what type of twins they are. Sometimes, twins grow up never knowing for sure whether they are identical twins or not.

At the time of birth, the physician will try to get an idea of whether twins are identical or fraternal by examining whether they share the same sac. Identical twins often form in a single sac, sharing a placenta and amniotic membranes. Fraternal twins often form in two separate sacs. However, up to 25% of the time, this method of determining twin type is not accurate and can be very misleading. In some cases, twins can have two sets of outer sacs (dichorionic) and still be identical twins. Also, the two placentas of fraternal twins can sometimes fuse together and appear to be one.

The only way to conclusively determine whether twins of the same sex are identical is through a simple DNA test. For this reason, many parents of twins will consider a private DNA test for twins to determine conclusively whether the twins are identical or fraternal. The DNA twin test is fast, simple and painless. A DNA sample is taken from each child by rubbing a painless swab gently inside the child’s mouth for ten seconds. The samples are then submitted to the laboratory for testing and conclusive results are available within 3 to 5 days.

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