The Multiple Birth Foundation explains:
“Are they identical” is one of the first questions people ask about twins or triplets. Parents and also the children themselves when they are older, will want to know. This is known as determining zygosity.
Non–identical twins arise when two separate eggs are fertilised by separate sperm and will have genes from both parents but are no more alike than any brothers and sisters. Identical twins arise when a single embryo divides and the babies then have the same genes.
When you are scanned early in pregnancy the sonographer will look to see whether your twins or triplets share one placenta. If so there will be a single outer membrane (the chorion) and the term monochorionic is used to describe your pregnancy. These babies will always be identical. However about one third of identical twins will have two separate placentas because their embryo divided very soon after it was fertilised and before the placenta started to form. When the babies are born, if they are different sexes they will of course be non-identical. If your babies are the same sex and you are not certain whether or not they are identical a simple test can be done to compare their DNA.”
(Note there is currently a charge for DNA testing in some hospitals.)
For more information the MBF has leaflets on how to tell whether twins are identical as well as zygosity testing on monochorionic twins. For more information contact the MBF on 020 313 3519 www.multiplebirths.org.uk