Identical Twins: The Rule of 4

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.

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.

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.

·         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.

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.