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Identical Twins: The Rule of 4

Updated on May 29, 2012

Most people will know how non-identical twins (fraternal twins, to give them their proper name) happen: Two eggs are released simultaneously, it happens to coincide with when sex takes place; both are fertilized and, voilà! You have two. Now, these twins could be as alike or as different as any other pair of siblings born years apart. They just happen to share the womb space and birthday (in most cases!). Crucially, in the womb, non-identical twins will always be in separate sacs and have two separate placentas. They never share the sac or placenta. For identical twins, things are not so straight forward.

Non-identical twins could be similar or different physically, just like any other set of siblings.
Non-identical twins could be similar or different physically, just like any other set of siblings. | Source

...and now for identical twins

Identical twins are a result of cleavage of a fertilized egg into two. It means, therefore, that the two resulting embryos will have exactly the same genetic material. Needless to say, they are always of the same sex and will have the characteristic physical likeness that never ceases to fascinate. Identical twins don’t end up being exactly the same in their adult lives because; personality is shaped both by genes and the environment. Don’t always expect identical twins to have the exact same outlook to life.

The rule of 4

Unlike their non-identical counterparts, identical twins fall into four distinct groups. There is ‘the rule of four’ taking shape.

·         Cleavage of the fertilized egg can take place in the first four days (after fertilization). The resulting twins will have separate placentas and separate amniotic sacs. Ultrasound scan images look the same as what you will see with non-identical twins. The medical term for this type of twins is dichorionic diamniotic. In every aspect, this pregnancy will be the same as what you will see with non-identical twins. In fact, with this type of twins, it may not be possible to know for certain whether they are identical or not until after the birth (unless they are different sex, in which case it is clear).

·         Cleavage can take place between Day 5 and Day 8. This results in twins that will share the placenta but will be in two separate amniotic sacs. These are called Monochorionic diamniotic. There is one potentially serious complication associated with this type of twins. It is called TTTS (twin to twin transfusion syndrome). Because of sharing the placenta, there could be communication between the two circulations which could result in one twin losing blood to the other. The one receiving is the twin at most peril because the cardiovascular system gets overwhelmed with the volume of blood. However, both twins are in danger and are often lost especially if the complication starts early in pregnancy and remote from the stage of viability.

Identical twins, clearly in the same sac. These will also be sharing a placenta.
Identical twins, clearly in the same sac. These will also be sharing a placenta.

·         Cleavage of the fertilized egg may delay and take place between day 9 and day 12. This results in the monochorionic monoamniotic twins, meaning these two will be sharing the placenta as well as the gestational sac. This conjures up an image of developing individuals who are very close and always in physical contact with one another from the earliest stage of their lives. While dynamic ultrasound scan images of such twins predictably provokes ‘Aaahs’ and ‘Ooohs’ at a picture of such intimacy in the womb, these twins are in the most peril. Apart from the potential problem of TTTS described above, they are also prone to other complications unique to them such as cord entanglement.

·         The final type of identical twins results from a much delayed cleavage taking place after day 12.  This will result in incomplete separation and therefore you end up with conjoined twins. Again, the degree of separation will depend on how late the process of cleavage starts taking place. Delivery is almost always by caesarean section. The challenge for conjoined twins normally comes after the birth. This takes the form of the feasibility of surgically separating them. Occasionally, even at birth, one of the twins is incompletely formed and not viable. In some cases, the join is such that it is not feasible to separate them without losing one or sometimes both.

Cleavage after Day 12. It may not be possible to surgically separate this type of conjoined twins
Cleavage after Day 12. It may not be possible to surgically separate this type of conjoined twins

So, to re-cap: There are four groups of identical twins (also called monozygotic twins). Division of the fertilized egg into two in the first four days results in twins with separate placentas and in separate sacs; in the next four days, twins will share a placenta but have separate sacs; the third lot of four days, twins will share both the placenta and the sac and beyond that, the separation will be incomplete ending up with conjoined twins.  Now you know.


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    • profile image


      6 years ago

      I found this information quite relovent to myself; as I am one of (of course) two identical twins. To see how I could have ended up the same as the conjoined twins in your picture is quite scary!!!!!!!!!!!! Personally I would like to know the outcome of THEIR story! Thanks.

    • akanga1 profile imageAUTHOR


      7 years ago

      Nixi, I thank you for your comments. Your definition of 'stupid' is clearly different from mine. Be that as it may be, I am guessing that the 'girl's story' you refer to in your comment is the one about the two sets of twins in the first image. If you check again, you will see that a link to the source of the image was helpfully included. Had you clicked on that, you would have found the story about these 'black and white' sets of twins. Again, thank you for stopping by.

    • profile image


      7 years ago

      I wanted to know the girl's story, not how identical & non identical twins are made. Like if the white & black girl are twins & if the white and black babies were twins. Stupid & disapointing.

    • akanga1 profile imageAUTHOR


      8 years ago

      Thank you for the kind words. Fraternal twins (also called dizygotic) incidence varies by region and ethnic groups. Incidence is lowest in Asia where it ranges around 4-6/1000 births. In Europe , reported rates range from 9 to 21/1000. In England alone it is 16/1000. In the United States, the averaged rate is 32/1000 births with Hispanics having the lowest rate at 22/1000. Whites and African Americans have a roughly similar rate at 36 and 37/1000 respectively. 1 in 6 of each of these is due to assisted conception such as IVF. Fraternal twins frequency is highest in Africa and in particular the Yoruba ethnic group in Nigeria with a frequency of 45-50 per1000 births. Fraternal twins are largely familial, on the mother’s side because the underlying driver is the release of multiple eggs. A male partner cannot influence that. In the developed world, rates of fraternal twins have been influenced by assisted conception which increases the possibility of twins as well as women having children later in life. A woman over the age of 35 has twice the possibility of having twins compare to a mother in her early or mid 20s.

      Monozygotic (identical) twins rate are the same throughout the world at 3- 4/1000. The vast majority are sporadic, meaning they occur by chance rather than genetic predisposition. There is a small subset of identical twins where there is evidence of a yet to be identified genetic predisposition. In addition, with the genetic disorder known as Beckwith-Wiedemann Syndrome, the possibility of having identical twins is increased almost 2-fold. This is a rare condition.

      As to why the mother does not reject the fetus, that’s an interesting question. To do it justice, the answer cannot be short. I will therefore create a separate hub to answer that. Suffice it to summarise here that there is a complex mechanism of down-regulation of the immune response to foreign tissue at the placental interface to ensure no rejection takes place.

    • smcopywrite profile image


      8 years ago from all over the web

      thank you soooo much for this information. this is the most in depth and understandable explanation of identical twins and why you receive what you do.days and hours can make a lifetime of difference

      the human body is amazing and marvelous and you are always afraid of the smallest thing going wrong because of the complicating factors that are taking place at all times. fertility is as complicated as it gets because you are producing a replica of the human body.

      do you have any data on twins and the rate of identical and fraternal twins related to country, race or nationality?-without infertility drug or procedure help

      i have one more question for you. if the human body will reject anything foreign, why doesn't the female body reject this object with foreign DNA?


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