The Differences Between Twins

Identical Twins (also known as Uniovular or Monozygotic twins). 

During the fertilization process, only one ovum is impregnated by only one spermatozoid, but the egg divides in 2 embryos afterwards. They have exactly the same chromosomes, they have a perfect resemblance and they have the same sex.

There are 3 different ways that identical twins can develop and be carried: 

• With the first case there is only one placenta that feeds the babies, but there can be two amniotic sacs (as in the diagram at the side).  When there is 1 placenta and 2 amniotic sacs then the pregnancy is referred to as having a "mono-chorial" placenta and is "bi-amniotic".

• In the second scenario, there is only one amniotic sac.  In the case of 1 placenta and 1 amniotic sac then the pregnancy is referred to as having a "mono-chorial" placenta and is "mono-amniotic".

• The third scenario is where there are two placentae and each embryo has its own amniotic pocket. When there are two placentas and two amniotic pockets like this, then one speaks of a "bi-chorial" pregnancy that is "bi-amniotic". (This last case is an identical situation to fraternal twins as shown below).

Non-Identical Twins (also known as Fraternal, Binovular or Dyzygotic twins). 

When twins are non-identical, then two seperate eggs have been impregnated by 2 separate spermatozoa.

They each have a different chromosome make-up, they don't look alike and they can be either the same or different sex.  In fact, this is simply two separate pregnancies that just happened to occur at the same time.

Each embryo is in its own amniotic sac and has its own placenta.  This type of pregnancy is referred to as "bi-chorial" and "bi-amniotic".

What's happening in each scenario? 

The Monozygotic twins are derived from only one egg and only one fertilization.

Scenario 1: If the separation takes place just after the first cellular division, then in this case both of the twins will have their own placenta and an amniotic sac each.

Scenario 2: The separation can also take place a little later in the development of the embryonic cells but before the blastocyte has fully defined the roles of each type of cell. At that moment, twins will be in the same placenta, but they will have 2 amniotic sacs.

Scenario 3: In the last of the identical twin scenarios, if the separation takes place at the stage when the amniotic bag is already being formed, then the twins will be in the same placenta, and in the same amniotic sac.

Scenario 4: Dyzygotic twins (fraternal twins), as shown in the diagram, are descended from a double ovulation and a double fertilization. The 2 eggs are completely independent. This configuration represents two thirds of all twin pregnancies.

Scenario 1 and Scenario 4 clearly have the same configuration at implantation into the uterus wall (bi-chorial and bi-amniotic) yet they are very different in that when the twins derive from one egg (monozygotic) they will be identical and the twins coming from two separate eggs will be fraternal.