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One egg or two?

Several Gemini families have told us their stories of the challenge they have experienced in trying to decide whether their twins are identical or fraternal (non-identical). Strange as it may sound, this is a common problem and it can be very hard both for parents and health professionals to identify 'twin type' without using a DNA test.

Identical twins occur when a single fertilized egg splits in to two halves, and makes two babies with 100% of the same genes. This means that identical twins are always the same sex and generally look very similar. However, identical twins are not the same in every way (as many Gemini parents will be quick to point out) and can show differences in appearance, personality and abilities.

Fraternal twins come from two different eggs that are fertilized by two different sperm. This means that genetically, fraternal twins are the same as ordinary brothers and sisters, who share on average 50% of the same genes. However, because the genes we inherit from our parents are a matter of chance, some fraternal twin pairs can share more genes, whilst other twins share less. This means that some fraternal twins can act and look quite different, whilst other pairs are very similar.

Unless your babies are a boy and a girl it can be very difficult to tell whether twins are truly genetically identical. Fraternal twins (that are the same sex) can sometimes look very similar and identical twins can sometimes look very different. Parents know their children so well and are so sensitive to their differences that some parents might wrongly decide that identical twins are fraternal. Alternatively some fraternal same-sex twins may look so similar that people constantly mistake them for being identical.

James, Emily and siblings
(a) Fraternal or identical twins with separate placentas and separate amniotic sacs

James, Emily and siblings
(b) Identical twins with a shared placenta and a shared amniotic sac

James, Emily and siblings
(c) Identical or fraternal twins with a single or fused placenta and separate amniotic sacs

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We have received accounts from Gemini families who were told their twins were definitely fraternal because they each had their own placenta and amniotic sac. Whilst it is true that all fraternal twins have their own placenta and sac, so do one third of all identical twins. Therefore, using this information to identify 'twin type' can lead to many sets of identical twins wrongly being classified as fraternal. To complicate matters further, it is possible for the two placentae of fraternal twins to fuse together and become one, causing some same-sex fraternal twins to be mistaken for identical.

Gemini mother Lucinda told us about her experiences of trying to find out whether or not her sons Oliver and Joseph were identical:

Oliver and Joseph

"During my pregnancy I was always told that my twins were definitely non- identical because they had 2 sacs and 2 placentas. However for over 9 months myself and family members often muddled them up. I therefore decided to have my twins' DNA tested to clear up all the constant debates with family, friends and strangers!
It turned out that Oliver and Joseph are in fact identical. We felt angry and let down by the health professionals, who even after they were born, told us they couldn't be identical due to their separate sacs and placentas. I felt confused, as if I didn't really know my children or what my body had done. We only feel that parents should be warned not to assume 'non-identicalness' until tested."

There are several reliable procedures for working out if same-sex twins are identical or fraternal, including so called "physical resemblance questionnaires". These questionnaires focus on identifying differences in physical characteristics, such as eye and hair colour, that are determined by our genes. You may remember that the first Gemini questionnaire included some questions about how similar or different your twins look. The classification of twin type from these questionnaires is very accurate and matches with the results of DNA tests 95% of the time.

Whilst physical resemblance questionnaires are a reliable method of identifying twins as either identical or fraternal, there remain some twin pairs who are not easily grouped using this method. In these cases only a DNA test can provide a definite answer to the question 'did my twins come from one egg or two?'