Identical
Identical twins (also called monozygotic) occur when a single egg is fertilised and then splits in half. These twins will be the same sex and will usually look very similar to each other. They have the same DNA or genetic material as each other. Some times the only way to tell if they are identical or not is by a dna test.
- If the egg splits within the first three days after fertilisation then the twins will be dichorionic, this means that they will each have their own sac with seperate placentas.
- If the egg splits between three and nine days after fertilisation then the twins will be monochorionic and diamniotic, which means that they will share a placenta but still be in two seperate sacs.
- If the egg splits between nine and twelve days after fertilisation the the twins will be monochorionic and monoamniotic, meaning that they will share a sac and a placenta.
- If the egg splits after twelve days there is a chance that the twins could be conjoined (siamese).
Monoamniotic twins are identical twins which develop inside the same amniotic sac. They have separate umbilical cords; however they share the same placenta. They are also known as MoMo twins (Monoamniotic-Monochorionic).
Mono Mono twins Ireland – article by Sharon Keogh.
Non-identical
Non-Identical twins (also called dizygotic or fraternal) are much more common than identical twins. They occur when two separate eggs are fertilised by two separate sperms. One half of non-identical twins result in boy/girl twins.
There are many theories as to why twins occur but despite a huge amount of research, the exact cause is unknown. However, non-identical twins are more likely to occur in women that:
- Are above thirty
- Are of African origin
- Have a history of twins in their family
- Have children already
- Have good nutrition
- Have increased fertility
- Have undergone fertility treatment
- Have used the contraceptive pill
If you are unsure whether your multiples are identical or not, you can do a dna test to find out. This is the most accurate way of checking. www.dnaireland.ie offers IMBA members a 30% discount on dna testing. If the egg splits very early, identical twins can have separate sacs and separate placentas which often confuses people into thinking they are non identical. This has happened to many of our members and only a dna test has proved they are in fact identical. Also the placentas can fuse and may appear the twins are identical when in fact they are non identical. The dna test is very simple, a swob of the inside of the cheek of the multiples.