Fraternal twins develop from two different eggs, each fertilised by its own sperm cell. Before birth each fraternal twin is nourished by its own placenta. Because the two twins develop from a different egg and sperm cell, they are in fact like ordinary siblings — they may or may not look alike, and may or may not be of the same sex. Their genetic make-up is not identical.
Identical twins develop from a single egg and a single sperm cell, but the embryo then splits in two. Identical twins therefore usually share a placenta, have the same sex and share more than 95 % of their DNA. In a quarter of identical twins, however, the split happens within three days of fertilisation and each twin then has its own placenta, much as with fraternal twins. Unlike fraternal twins, though, their genetic make-up is identical. Comparing the DNA of both twins therefore lets us determine their zygosity conclusively.
The result of a twin zygosity test consists of the test identifiers (case number, sample numbers), a listing of the 17 DNA markers tested and a written evaluation of the result, including confirmation or refutation of identical twinning. You will receive the results within 5 working days as standard.